gh-82665 Mention that HTMLParser.handle_starttag value can be None (#134312)

* Specify boolean attribute behavior in parser

* Tweak wording and example

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>

* Fix backticks

---------

Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>