Add documentation for confidential notes

Confidential param can be set for epics via API
Confidential notes are only supported for issues
and epics

doc/api/notes.md

@@ -519,11 +519,12 @@ POST /groups/:id/epics/:epic_id/notes
 
 Parameters:
 
-| Attribute | Type           | Required | Description |
-| --------- | -------------- | -------- | ----------- |
-| `id`      | integer or string | yes | The ID or [URL-encoded path of the group](index.md#namespaced-path-encoding) |
-| `epic_id` | integer | yes  | The ID of an epic |
-| `body`    | string  | yes  | The content of a note. Limited to 1,000,000 characters. |
+| Attribute      | Type           | Required | Description |
+| ---------      | -------------- | -------- | ----------- |
+| `body`         | string  | yes  | The content of a note. Limited to 1,000,000 characters. |
+| `epic_id`      | integer | yes  | The ID of an epic |
+| `id`           | integer or string | yes | The ID or [URL-encoded path of the group](index.md#namespaced-path-encoding) |
+| `confidential` | boolean        | no       | The confidential flag of a note. Default is `false`. |
 
 ```shell
 curl --request POST --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/groups/5/epics/11/notes?body=note"

doc/user/discussions/index.md

@@ -152,7 +152,8 @@ If an issue or merge request is locked and closed, you cannot reopen it.
 
 ## Mark a comment as confidential **(FREE SELF)**
 
-> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/207473) in GitLab 13.9 [with a flag](../../administration/feature_flags.md) named `confidential_notes`. Disabled by default.
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/207473) in GitLab 13.9 [with a flag](../../administration/feature_flags.md) named `confidential_notes`. Disabled by default.
+> - [Changed](https://gitlab.com/gitlab-org/gitlab/-/issues/351143) in GitLab 14.10: you can only mark comments in issues and epics as confidential. Previously, it was also possible for comments in merge requests and snippets.
 
 FLAG:
 On self-managed GitLab, by default this feature is not available. To make it available,
@@ -160,9 +161,25 @@ ask an administrator to [enable the feature flag](../../administration/feature_f
 On GitLab.com, this feature is not available.
 You should not use this feature for production environments.
 
-You can make a comment confidential, so that it is visible only to project members
-who have at least the Reporter role.
+You can make a comment **in an issue or an epic** confidential, so that it is visible only to you (the commenting user) and
+the project members who have at least the Reporter role.
 
+Keep in mind:
+
+- You can only mark comments as confidential when you create them.
+- You can't change the confidentiality of existing comments.
+- Replies to comments use same confidentiality as the original comment.
+
+Prerequisites:
+
+- You must either:
+  - Have at least the Reporter role for the project.
+  - Be the issue assignee.
+  - Be the issue author.
+
+To mark a comment as confidential:
+
+1. Start adding a new comment.
 1. Below the comment, select the **Make this comment confidential** checkbox.
 1. Select **Comment**.