documentation: Catch up list of torture_type options

This commit rids the documentation of long-obsolete torture_type options
such as rcu_sync and adds new ones such as tasks.  Also add verbiage
noting the fact that rcutorture now concurrently tests the asynchrounous,
synchronous, expedited synchronous, and polling grace-period primitives.

Reported-by: David Miller <davem@davemloft.net>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: David Miller <davem@davemloft.net>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>