rcu: Update documentation of rcu_read_unlock()

rcu: Update documentation of rcu_read_unlock()

Since commit b4abf91047cf ("rtmutex: Make wait_lock irq safe") the
explanation in rcu_read_unlock() documentation about irq unsafe rtmutex
wait_lock is no longer valid.

Remove it to prevent kernel developers reading the documentation to rely on
it.

Suggested-by: Eric W. Biederman <[email protected]>
Signed-off-by: Anna-Maria Gleixner <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]>
Acked-by: "Eric W. Biederman" <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]