added API docs

CHANGELOG
improved comments in API code
improved spec description

changelogs/unreleased/ide-merge-request-info.yml

+---
+title: Display merge request title & description in Web IDE
+merge_request:
+author:
+type: added

doc/api/merge_requests.md

@@ -358,6 +358,7 @@ Parameters:
 
 - `id` (required) - The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user
 - `merge_request_iid` (required) - The internal ID of the merge request
+- `render_html` (optional) - If `true` response includes rendered HTML for title and description
 
 ```json
 {

lib/api/merge_requests.rb

@@ -232,7 +232,7 @@ module API
 
       params do
         requires :merge_request_iid, type: Integer, desc: 'The IID of a merge request'
-        optional :render_html, type: Boolean, desc: 'Returns the description and title rendered html'
+        optional :render_html, type: Boolean, desc: 'Returns the description and title rendered HTML'
       end
       desc 'Get a single merge request' do
         success Entities::MergeRequest

spec/requests/api/merge_requests_spec.rb

@@ -306,7 +306,7 @@ describe API::MergeRequests do
       expect(json_response['changes_count']).to eq(merge_request.merge_request_diff.real_size)
     end
 
-    it 'exposes description and title html' do
+    it 'exposes description and title html when render_html is true' do
       get api("/projects/#{project.id}/merge_requests/#{merge_request.iid}", user), render_html: true
 
       expect(response).to have_gitlab_http_status(200)