doc: say "compatible types" for -fstrict-aliasing

doc: say "compatible types" for -fstrict-aliasing

Include the term used in the standard to ease further research for users,
and while at it, rephrase the description of the rule entirely using
Alexander Monakov's suggestion: it was previously wrong (and imprecise) as
"the same address" may well be re-used later on, and the issue is the
access via an expression of the wrong type.

gcc/ChangeLog:

	* doc/invoke.texi: Use "compatible types" term. Rephrase to be
	more precise (and correct).