srcu: Fix srcu_read_unlock_{lite,nmisafe}() kernel-doc

The srcu_read_unlock_lite() and srcu_read_unlock_nmisafe() both say that
their idx parameters must come from srcu_read_lock().  This would be bad,
because a given srcu_struct structure may be used only with one flavor of
SRCU reader.  This commit therefore updates the srcu_read_unlock_lite()
kernel-doc header to say that its idx parameter must be obtained
from srcu_read_lock_lite() and the srcu_read_unlock_nmisafe()
kernel-doc header to say that its idx parameter must be obtained from
srcu_read_lock_nmisafe().

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>