📚 Document fields for BodyType structs as methods

* Move struct field docs from the class to methods inside the class.
* Remove ABNF comments.
* Add type information to the call-seq or the method rdoc text.
* Link to the data type(s) that are returned, when possible.

Add warnings to BodyTypeAttachment and BodyTypeExtension.  BodyTypeText
and BodyTypeMessage link to BodyTypeBasic for documentation of common
fields.  Link to relevant RFCs where relevant.  Update formatting.