openmp: Add documentation for omp_{get_max, set_num}_threads and omp_{s, g}et_teams_thread_limit

This patch adds documentation for these new OpenMP 5.1 APIs as well as
two new environment variables - OMP_NUM_TEAMS and OMP_TEAMS_THREAD_LIMIT.

2021-10-12  Jakub Jelinek  <jakub@redhat.com>

	* libgomp.texi (omp_get_max_teams, omp_get_teams_thread_limit,
	omp_set_num_teams, omp_set_teams_thread_limit, OMP_NUM_TEAMS,
	OMP_TEAMS_THREAD_LIMIT): Document.