Merge branch 'improve-oauth2-docs' into 'master'

Update/Improve OAuth2 Client documentation

## What does this MR do?
Improves the OAuth2 client documentation to include security hints to help people create secure clients.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?
I noticed the docs didn't mention some important points and steps, this cleans that up.

## What are the relevant issue numbers?

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5665

CHANGELOG

@@ -71,6 +71,7 @@ v 8.11.0 (unreleased)
   - Add GitLab Workhorse version to admin dashboard (Katarzyna Kobierska Ula Budziszewska)
   - Allow branch names ending with .json for graph and network page !5579 (winniehell)
   - Add the `sprockets-es6` gem
+  - Improve OAuth2 client documentation (muteor)
   - Multiple trigger variables show in separate lines (Katarzyna Kobierska Ula Budziszewska)
   - Profile requests when a header is passed
   - Avoid calculation of line_code and position for _line partial when showing diff notes on discussion tab.

doc/api/oauth2.md

 # GitLab as an OAuth2 client
 
-This document is about using other OAuth authentication service providers to sign into GitLab.
-If you want GitLab to be an OAuth authentication service provider to sign into other services please see the [Oauth2 provider documentation](../integration/oauth_provider.md).
+This document covers using the OAuth2 protocol to access GitLab.
 
-OAuth2 is a protocol that enables us to authenticate a user without requiring them to give their password. 
+If you want GitLab to be an OAuth authentication service provider to sign into other services please see the [Oauth2 provider documentation](../integration/oauth_provider.md).
 
-Before using the OAuth2 you should create an application in user's account. Each application gets a unique App ID and App Secret parameters. You should not share these.
+OAuth2 is a protocol that enables us to authenticate a user without requiring them to give their password to a third-party. 
 
 This functionality is based on [doorkeeper gem](https://github.com/doorkeeper-gem/doorkeeper)
 
 ## Web Application Flow
 
-This flow is using for authentication from third-party web sites and is probably used the most. 
-It basically consists of an exchange of an authorization token for an access token. For more detailed info, check out the [RFC spec here](http://tools.ietf.org/html/rfc6749#section-4.1)
+This is the most common type of flow and is used by server-side clients that wish to access GitLab on a user's behalf.
+
+>**Note:**
+This flow **should not** be used for client-side clients as you would leak your `client_secret`. Client-side clients should use the Implicit Grant (which is currently unsupported).
 
-This flow consists from 3 steps.
+For more detailed information, check out the [RFC spec](http://tools.ietf.org/html/rfc6749#section-4.1)
+
+In the following sections you will be introduced to the three steps needed for this flow.
 
 ### 1. Registering the client
 
-Create an application in user's account profile.
+First, you should create an application (`/profile/applications`) in your user's account.
+Each application gets a unique App ID and App Secret parameters. 
+
+>**Note:**
+**You should not share/leak your App ID or App Secret.**
 
 ### 2. Requesting authorization
 
-To request the authorization token, you should visit the `/oauth/authorize` endpoint. You can do that by visiting manually the URL:
+To request the authorization code, you should redirect the user to the `/oauth/authorize` endpoint:
+
+```
+https://gitlab.example.com/oauth/authorize?client_id=APP_ID&redirect_uri=REDIRECT_URI&response_type=code&state=your_unique_state_hash
+```
+
+This will ask the user to approve the applications access to their account and then redirect back to the `REDIRECT_URI` you provided.
+
+The redirect will include the GET `code` parameter, for example:
 
 ```
-http://localhost:3000/oauth/authorize?client_id=APP_ID&redirect_uri=REDIRECT_URI&response_type=code
+http://myapp.com/oauth/redirect?code=1234567890&state=your_unique_state_hash
 ```
 
-Where REDIRECT_URI is the URL in your app where users will be sent after authorization. 	
+You should then use the `code` to request an access token.
+
+>**Important:**
+It is highly recommended that you send a `state` value with the request to `/oauth/authorize` and 
+validate that value is returned and matches in the redirect request. 
+This is important to prevent [CSFR attacks](http://www.oauthsecurity.com/#user-content-authorization-code-flow), 
+`state` really should have been a requirement in the standard! 
 
 ### 3. Requesting the access token
 
-To request the access token, you should use the returned code and exchange it for an access token. To do that you can use any HTTP client. In this case, I used rest-client:
+Once you have the authorization code you can request an `access_token` using the code, to do that you can use any HTTP client. In the following example, we are using Ruby's `rest-client`:
 
 ```
 parameters = 'client_id=APP_ID&client_secret=APP_SECRET&code=RETURNED_CODE&grant_type=authorization_code&redirect_uri=REDIRECT_URI'
@@ -46,6 +67,8 @@ RestClient.post 'http://localhost:3000/oauth/token', parameters
  "refresh_token": "8257e65c97202ed1726cf9571600918f3bffb2544b26e00a61df9897668c33a1"
 }
 ```
+>**Note:**
+The `redirect_uri` must match the `redirect_uri` used in the original authorization request.
 
 You can now make requests to the API with the access token returned.
 
@@ -77,6 +100,9 @@ The credentials should only be used when there is a high degree of trust between
 client is part of the device operating system or a highly privileged application), and when other authorization grant types are not
 available (such as an authorization code).
 
+>**Important:**
+Never store the users credentials and only use this grant type when your client is deployed to a trusted environment, in 99% of cases [personal access tokens] are a better choice.
+
 Even though this grant type requires direct client access to the resource owner credentials, the resource owner credentials are used
 for a single request and are exchanged for an access token.  This grant type can eliminate the need for the client to store the
 resource owner credentials for future use, by exchanging the credentials with a long-lived access token or refresh token.