gh-129873: IDLE: Improve help.py's method of parsing HTML (#129859)

In `help.copy_strip`, only copy the text `<section>`.  In `help.HelpParser.handle_starttag` and elsewhere, remove code to skip the no longer present html.  Add a reminder at the top of idle.rst to run copy_strip after changes.
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Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>