libgomp.texi: Fix ICV var name, document some memory management routines

libgomp/

	* libgomp.texi (Memory Management Routines): New; add documentation for
	omp_init_allocator, omp_destroy_allocator, omp_set_default_allocator,
	omp_get_default_allocator.
	(OMP_ALLOCATOR): Fix ICV var name; add see-also references.