Documentation: update path-lookup.md for parallel lookups

Since this document was written, i_mutex has been replace with
i_rwsem, and shared locks are utilized to allow lookups in the one
directory to happen in parallel.

So replace i_mutex with i_rwsem, and explain how this is used for
parallel lookups.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>