Improve documentation for rich text support in the API.

This introduces a new page detailing how the rich text support works in
the API. It covers the text types, entries in extra_data, forcing text
types, and more.

All existing resources have had their custom docs on rich text support
removed in favor of the new page. These resources also now have the
"supports_text_types" flag set for the POST/PUT fields.

The docs generator uses this flag when building the request/response
field tables. If set, it will override the field's text type to
"Rich Text," and link to the new documentation.

Reviewed at https://reviews.reviewboard.org/r/6535/