Verbose Header Object documentation (3.0.4)

[handrews]

Fixes:

• Documentation for OpenAPI's Header Object should be verbose #1965

This copies the relevant Parameter Object fields to the Header Object instead of relying on implicit guidance. The text for the fields has been edited to reflect that only headers are being described.

This also include an example of describing a header using the content field, and explaining why it is necessary to do so.

NOTE: Includes some appendix headings to placate the PR markdown validation check.

[ralfhandl]

Thanks for the diligence!

Only minor suggestions.

[lornajane]

More information is good information IMO, thank you for this addition.

[ralfhandl]

Minor nit

[handrews]

This has two TSC approvals and a third positive-comment-with-questions-that-have-been-resolved near-approval so I'm going to merge this to start to reduce the amount of conflicts with empty sections and the like.