documentation.

Suggested by Skip Montanaro on docs@.

for audio modules: aifc, sunau and wave.

Closes #14486: add versionchanged notices throughout the threading docs for PEP8-compliant APIs; the note at the top of the page is too hard to notice.

emulation has changed from 0-based indexing to 1-based like gnu readline.
Original patch by Ronald Oussoren.

failed, second invocation of close() and destructor no more raise an
exception.  Second invocation of close() on sunau writer now has no effects.
The aifc module now accepts lower case of names of the 'ulaw' and 'alaw'
codecs.

Closes #13905: mention rich-comparison methods in addition to __cmp__ when documenting how to make classes comparable and orderable.

Closes #19179: make table of XML vulnerabilities clearer by using "everyday" booleans and explaining the table beforehand.

compressed streams.

(grafted from e06edc0c7a4951327f0c95ebeebccba6879a6063)