doc/implement-c.texi: About same-as-scalar-type volatile aggregate accesses, PR94600

doc/implement-c.texi: About same-as-scalar-type volatile aggregate accesses, PR94600

We say very little about reads and writes to aggregate /
compound objects, just scalar objects (i.e. assignments don't
cause reads).  Let's lets say something safe about aggregate
objects, but only for those that are the same size as a scalar
type.

There's an equal-sounding section (Volatiles) in extend.texi,
but this seems a more appropriate place, as specifying the
behavior of a standard qualifier.

gcc:

2020-12-04  Hans-Peter Nilsson  <hp@axis.com>
	    Martin Sebor  <msebor@redhat.com>

	PR middle-end/94600
	* doc/implement-c.texi (Qualifiers implementation): Add blurb
	about access to the whole of a volatile aggregate object, only for
	same-size as a scalar object.

(cherry picked from commit eb79f4db49c5f5a807555e9d374524664eb537bf)