doc/api/commits: Add /projects/:id/repository/discussions documentation

example:
- https://gitlab.com/api/v4/projects/gitlab-org%2Fgitlab/repository/commits/695c29abcf7dc2eabde8d59869abcea0923ce8fa/discussions

doc/api/commits.md

@@ -521,6 +521,62 @@ Example response:
 }
 ```
 
+## Get the discussions of a commit
+
+Get the discussions of a commit in a project.
+
+```plaintext
+GET /projects/:id/repository/commits/:sha/discussions
+```
+
+Parameters:
+
+| Attribute | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `id`      | integer/string | yes | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user
+| `sha`     | string | yes | The commit hash or name of a repository branch or tag |
+
+```shell
+curl --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/projects/5/repository/commits/4604744a1c64de00ff62e1e8a6766919923d2b41/discussions"
+```
+
+Example response:
+
+```json
+[
+  {
+    "id": "4604744a1c64de00ff62e1e8a6766919923d2b41",
+    "individual_note": true,
+    "notes": [
+      {
+        "id": 334686748,
+        "type": null,
+        "body": "I'm the Dude, so that's what you call me.",
+        "attachment": null,
+        "author" : {
+          "id" : 28,
+          "name" : "Jeff Lebowski",
+          "username" : "thedude",
+          "web_url" : "https://gitlab.example.com/thedude",
+          "state" : "active",
+          "avatar_url" : "https://gitlab.example.com/uploads/user/avatar/28/The-Big-Lebowski-400-400.png"
+        },
+        "created_at": "2020-04-30T18:48:11.432Z",
+        "updated_at": "2020-04-30T18:48:11.432Z",
+        "system": false,
+        "noteable_id": null,
+        "noteable_type": "Commit",
+        "resolvable": false,
+        "confidential": null,
+        "noteable_iid": null,
+        "commands_changes": {}
+      }
+    ]
+  }
+]
+
+```
+
 ## Commit status
 
 Since GitLab 8.1, this is the new commit status API.