Markup tips for Markdown shown while editing wiki pages in other formats

What does this MR do?

We support creating wiki in RDoc / Asciidoc / Org formats, but there a few issues which prevent the process from being as smooth as creating wiki in Markdown:

• In the wiki editor we say:

  To link to a (new) page, simply type [Link Title](page-slug)

This syntax does not work for RDoc (see gitlab-ce#18287). A working syntax for RDoc is here[link:additional-page] (http://stackoverflow.com/questions/2230954/is-there-any-way-to-easily-link-a-file-in-rdoc)

This MR adds correct hints how to create a link when non-Markdown formats are selected in the dropdown.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Issue: #15103 (closed)