Add section titles, fix levels (3.1.1 port of 3923 2/6)

Make the Parameter, Encoding, and Header Object fixed fields
section organization the same in all three places, with the
same levels of indentation.

Add more headings under the Encoding Object for guidance on
each form media type, and sub-headings for each example in
each of those sections.

This will make the diff for the next commit much more legible.