Fix HTML validation errors - remove h7 tag, document the rule in FDP

""Listening Side"" and ""Initiating Side"" in the Developers Handbook
used level 5 headings, which render as <h7> in single-page HTML
output for books and fail W3C validation.

Replace them with a bold phrase, and add a WARNING to the
Asciidoctor chapter of the FDP Primer that level 5 headings are
unusable in books.

Reviewed by: carlavilla
Approved by: carlavilla
Differential Revision: https://reviews.freebsd.org/D60070
PR: 297509