Docs: Address -fivopts, -O0, and -Q confusion [PR71094]

There's a blurb at the top of the "Optimize Options" node telling
people that most optimization options are completely disabled at -O0
and a similar blurb in the entry for -Og, but nothing at the entry for
-O0.  Since this is a continuing point of confusion it seems wise to
duplicate the information in all the places users are likely to look
for it.

gcc/ChangeLog
	PR tree-optimization/71094
	* doc/invoke.texi (Optimize Options): Document that -fivopts is
	enabled at -O1 and higher.  Add blurb about -O0 causing GCC to
	completely ignore most optimization options.

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