Merge branch 'dz-mr-version-compare' into 'master'

Allow compare merge request versions

## What does this MR do?

Add new functionality to the merge request page. It allows you easily compare merge request versions in one click.  

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

To improve code review experience. 

## Screenshots (if relevant)

See discussion

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] ~~API support added~~
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/13570

See merge request !6127

1 files changed, 6 insertions, 0 deletions

diff --git a/doc/user/project/merge_requests/versions.md b/doc/user/project/merge_requests/versions.md
index 2b1c5ddd562..a6aa4b47835 100644
--- a/doc/user/project/merge_requests/versions.md
+++ b/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+Please note that comments are disabled while viewing outdated merge versions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:**