documentation.

Suggested by Skip Montanaro on docs@.

wave.

This does not fix #19227, but should stop the Gentoo buildbot from hanging.

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.

Initial wording proposed by John Roth.

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