Documentation: Add links to GitLab markdown documentation

The "Review a Merge Request" section primarily depends on comments made
in GitLab.  Link from this section to the GitLab markdown documentation.
In particular, the special reference syntax options will be very
helpful.

Documentation/dev/git/develop.md

@@ -209,6 +209,13 @@ draw their attention and have the topic reviewed.  After typing `@` and
 some text, GitLab will offer completions for developers whose real names
 or user names match.
 
+Comments use [GitLab Flavored Markdown][] for formatting.  See GitLab
+documentation on [Special GitLab References][] to add links to things
+like merge requests and commits in other repositories.
+
+[GitLab Flavored Markdown]: https://gitlab.kitware.com/help/markdown/markdown
+[Special GitLab References]: https://gitlab.kitware.com/help/markdown/markdown#special-gitlab-references
+
 ### Human Reviews ###
 
 Developers may add comments providing feedback or to acknowledge their