Doc: Clarify use of access attribute [PR101440]

Doc: Clarify use of access attribute [PR101440]

This issue was specifically about a confusing mention of the "second
and third arguments to the memcpy function" when only the second one
is a pointer affected by the attribute, but reading through the entire
discussion I found other things confusing as well; e.g. in some cases
it wasn't clear whether the "arguments" were the arguments to the
attribute or the function, or exactly what a "positional argument"
was.  I've tried to rewrite that part to straighten it out, as well as
some light copy-editing throughout.

gcc/ChangeLog
	PR c/101440
	* doc/extend.texi (Common Function Attributes): Clean up some
	confusing language in the description of the "access" attribute.