[3.13] gh-82665 Mention that HTMLParser.handle_starttag value can be None (GH-134312...

[3.13] gh-82665 Mention that HTMLParser.handle_starttag value can be None (GH-134312) (#149038)

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

* Specify boolean attribute behavior in parser

* Tweak wording and example

* Fix backticks

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(cherry picked from commit 804c213c89366dd5ffa7feeb1bd4feccfee75b38)

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