Add documentation for discussion lock

doc/user/discussions/index.md

@@ -153,12 +153,42 @@ comments in greater detail.
 
 ![Discussion comment](img/discussion_comment.png)
 
+## Locking discussions
+
+> [Introduced][ce-14531] in GitLab 10.1.
+
+When the discussion of an issue or a merge request is locked only team members can write new comments and edit the old ones.
+
+### Modifying discussion lock
+
+To lock or unlock a dicsussion, find the Lock section in the sidebar and click **Edit**. A popup should appear and give you the option to turn on or turn off the discussion lock.
+
+| Turn off discussion lock | Turn on discussion lock |
+| :-----------: | :----------: |
+| ![Turn off discussion lock](img/turn_off_lock.png) | ![Turn on discussion lock](img/turn_on_lock.png) |
+
+Every change is indicated by a system note in the issue's or merge request's comments.
+
+![Discussion lock system notes](img/discussion_lock_system_notes.png)
+
+### Indications of locked issues or merge requests
+
+While you are inside the locked issue or merge request and you are a project team member you can see an indicator in the comment form that the issue or merge request is locked.
+
+If you are not a team member you won't see the comment form but only the information that the dicussion is locked.
+
+| Team member | Not a member |
+| :-----------: | :----------: |
+| ![Comment form member](img/lock_form_member.png) | ![Comment form non-member](img/lock_form_non_member.png) |
+
+
 [ce-5022]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5022
 [ce-7125]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7125
 [ce-7527]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7527
 [ce-7180]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7180
 [ce-8266]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8266
 [ce-14053]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14053
+[ce-14531]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14531
 [resolve-discussion-button]: img/resolve_discussion_button.png
 [resolve-comment-button]: img/resolve_comment_button.png
 [discussion-view]: img/discussion_view.png