libgomp.texi: Update "Enabling OpenMP" + OpenACC / invoke.texi: -fopenacc/-fopenmp...

libgomp.texi: Update "Enabling OpenMP" + OpenACC / invoke.texi: -fopenacc/-fopenmp update

The OpenACC specification does not mention the '!$ ' sentinel for conditional
compilation and the feature was removed in r11-5572-g1d6f6ac693a860
for PR fortran/98011; update libgomp.texi for this and update a leftover
comment. - Additionally, some other updates are done as well.

libgomp/
	* libgomp.texi (Enabling OpenMP): Update for C/C++ attributes;
	improve wording especially for Fortran; mention -fopenmp-simd.
	(Enabling OpenACC): Minor cleanup; remove conditional compilation
	sentinel.

gcc/
	* doc/invoke.texi (-fopenacc, -fopenmp, -fopenmp-simd): Use @samp not
	@code; document more completely the supported Fortran sentinels.

gcc/fortran
	* scanner.cc (skip_free_comments, skip_fixed_comments): Remove
	leftover 'OpenACC' from comments about OpenMP's conditional
	compilation sentinel.