doc:lock: remove reference to clever use of read-write lock

Remove the clever example about read-write lock because this type of
lock is not recommended anymore (according to the very same document).
So there is no reason to teach clever things that people should not do.

Signed-off-by: Federico Vaga <federico.vaga@vaga.pv.it>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>