Modularity: Rich text formatting

[handrews]

Periodically, someone requests alternate or extended rich text formatting:

• Support for asciiDoc in the description attribute of the info and pathItem Object OpenAPI-Specification#1884 (@miqui, @briandominick, @ggrossetie)
• Provide guidance on Markdown table syntax OpenAPI-Specification#1867 (@shockey, @polarweasel)

The 3.x line really does not need further complexity right now, but as we re-imagine things for Moonwalk, it might be worthwhile to allow setting a rich text format rather than specifying one (that is a little under-specified for common usage like tables).

Alternatively, we could require the use of external documentation for such things, with the handling based on media types.