libgomp.texi: Clarify OMP_TARGET_OFFLOAD=mandatory

In OpenMP 5.0/5.1, the semantic of OMP_TARGET_OFFLOAD=mandatory was
insufficiently specified; 5.2 clarified this with extensions/clarifications
(omp_initial_device, omp_invalid_device, "conforming device number").
GCC's implementation matches OpenMP 5.2.

libgomp/ChangeLog:

	* libgomp.texi (OMP_DEFAULT_DEVICE): Update spec ref; add @ref to
	OMP_TARGET_OFFLOAD.
	(OMP_TARGET_OFFLOAD): Update spec ref; add @ref to OMP_DEFAULT_DEVICE;
	clarify MANDATORY behavior.