rcu: Upgrade srcu_read_lock() docbook about SRCU grace periods

It is illegal to wait for an SRCU grace period while within the
corresponding flavor of SRCU read-side critical section.  Therefore,
this commit updates the srcu_read_lock() docbook accordingly.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
1 file changed