gas: improve --gsframe documentation

I omitted documentation in 8aad677a12832885acd5be1de8f41e740b8e713d in
error. Rectify that with:
1) changing ---help to mention bare `--gsframe` too, as we're not
   getting rid of that;

2) adding the new --gsframe=[no|yes] form to as.texi.

	PR gas/33125
	* gas/as.c (parse_args): Tweak --gsframe= help text.
	* gas/doc/as.texi: Document --gsframe=[no|yes].