Documentation should note where GitLab-specific references are usable in Markdown

Our Markdown user documentation states:

You can use GitLab Flavored Markdown in:

• Comments
• Issues
• Epics
• Merge requests
• Milestones
• Snippets (the snippet must be named with a .md extension)
• Wiki pages
• Markdown documents inside repositories
• Releases

Note the inclusion of "Snippets" and "Markdown documents inside repositories".

It also lists extensions to standard Markdown that we offer, one of which is "GitLab-specific references".

These are not available in either Markdown documents inside repositories or .md snippets (essentially the same thing), and we should document this to avoid confusion.