libgomp.texi: Improve "OpenACC Environment Variables"

libgomp.texi: Improve "OpenACC Environment Variables"

None of the ACC_* env vars was documented; in particular, the valid valids
for ACC_DEVICE_TYPE found to be lacking as those are not document in the
OpenACC spec.
GCC_ACC_NOTIFY was removed as I failed to find any traces of it but the
addition to the documentation in commit r6-6185-gcdf6119dad04dd
("libgomp.texi: Updates for OpenACC.").  It seems to be planned as GCC
version of the ACC_NOTIFY env var used by another compiler for offloading
debugging.

libgomp/
	* libgomp.texi (ACC_DEVICE_TYPE, ACC_DEVICE_NUM, ACC_PROFLIB):
	Actually document what the function does.
	(GCC_ACC_NOTIFY): Remove unused env var.