rcu: Adjust the comment of function rcu_is_watching

Because RCU avoids interrupting idle CPUs, rcu_is_watching() is used to
test whether or not it is currently legal to run RCU read-side critical
sections on this CPU.  However, the first sentence and last sentences
of current comment for rcu_is_watching have opposite meaning of what
is expected.  This commit therefore fixes this header comment.

Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>