libgomp.texi: Document omp_display_env + omp_target_is_accessible

Additionally, it fixes a typo and changes the OpenMP 5.2 section
references (18.8.x) to OpenMP 5.1 ones (3.8.x) matching the mentioned
OpenMP number.

libgomp/ChangeLog:

	* libgomp.texi (OpenMP Technical Report 12): Fix a typo.
	(Device Memory Routines): Fix OpenMP 5.1 spec refs; add
	omp_target_is_accessible.
	(Environment Display Routine): Uncomment and add
	omp_display_env description.
	(OMP_DISPLAY_ENV): Update wording, add 'see also'.