Document extension metadata field.

This simple change adds a paragraph on how and why one would want to add the
``metadata`` attribute to their extension class.

Testing done: Generated HTML docs.
Reviewed at https://reviews.reviewboard.org/r/4794/
Add "Rich Text" fields in the administration UI.

This updates the admin UI forms for a few objects to provide "Rich Text"
fields. All the comments already have them, but these objects (Review,
ReviewRequest, ReviewRequestDraft) are all customizing the displayed
fields, and so need to explicitly provide them.
Format Markdown text for file attachment and screenshot comments.

Only diff comments were formatting Markdown text. Other comment types
ignored it completely and assumed it was plain text. Now it works for
all comment types.