Doc update: -foffload-options= examples + OpenMP in Fortran intrinsic modules

Doc update: -foffload-options= examples + OpenMP in Fortran intrinsic modules

With LTO, the -O.. flags of the host are passed on to the lto compiler, which
also includes offloading compilers. Therefore, using --foffload-options=-O3 is
misleading as it implies that without the default optimizations are used. Hence,
this flags has now been removed from the usage examples.

The Fortran documentation lists the content (except for API routines) routines
of the intrinsic OpenMP modules OMP_LIB and OMP_LIB_KINDS; this commit adds
two missing named constants and links also to the OpenMP 5.1 and 5.2
OpenMP spec for completeness.

gcc/ChangeLog:

	* doc/invoke.texi (-foffload-options): Remove '-O3' from the examples.

gcc/fortran/ChangeLog:

	* intrinsic.texi (OpenMP Modules OMP_LIB and OMP_LIB_KINDS): Also
	add references to the OpenMP 5.1 and 5.2 spec; add omp_initial_device
	and omp_invalid_device named constants.