Add changelog entry and documentation for merge request versions compare

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

CHANGELOG

@@ -118,6 +118,7 @@ v 8.11.4
   - Fix issue boards leak private label names and descriptions
   - Fix broken gitlab:backup:restore because of bad permissions on repo storage !6098 (Dirk Hörner)
   - Remove gitorious. !5866
+  - Allow compare merge request versions
 v 8.11.3
   - Allow system info page to handle case where info is unavailable

doc/user/project/merge_requests/versions.md

@@ -12,6 +12,12 @@ can select an older one from version dropdown.
 ![Merge Request Versions](img/versions.png)
+You can also compare the merge request version with older one to see what is
+changed since then.
+
+Please note that comments are disabled while viewing outdated merge versions
+or comparing to versions other than base.
+
 ---
 >**Note:**