Refactor merge requests revisions

- A system note now appears on every push
- Replace dashes with underscores in images

[ci skip]

doc/user/project/merge_requests/versions.md

@@ -7,26 +7,36 @@ of merge request diff is created. When you visit a merge request that contains
 more than one pushes, you can select and compare the versions of those merge
 request diffs.
-![Merge Request Versions](img/versions.png)
+![Merge request versions](img/versions.png)
+
+---
 By default, the latest version of changes is shown. However, you
 can select an older one from version dropdown.
-![Merge Request Versions](img/versions-dropdown.png)
-You can also compare the merge request version with older one to see what is
+![Merge request versions dropdown](img/versions_dropdown.png)
+
+---
+You can also compare the merge request version with an older one to see what has
 changed since then.
-![Merge Request Versions](img/versions-compare.png)
-Please note that comments are disabled while viewing outdated merge versions
-or comparing to versions other than base.
+![Merge request versions compare](img/versions_compare.png)
+
+---
+
+Every time you push new changes to the branch, a link to compare the last
+changes appears as a system note.
+![Merge request versions system note](img/versions_system_note.png)
 ---
->**Note:**
-Merge request versions are based on push not on commit. So, if you pushed 5
-commits in a single push, it will be a single option in the dropdown. If you
-pushed 5 times, that will count for 5 options.
+>**Notes:**
+- Comments are disabled while viewing outdated merge versions or comparing to
+  versions other than base.
+- Merge request versions are based on push not on commit. So, if you pushed 5
+  commits in a single push, it will be a single option in the dropdown. If you
+  pushed 5 times, that will count for 5 options.
 [ce-5467]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5467