memcg-v1: remove folio_memcg_lock() doc reference

memcg-v1: remove folio_memcg_lock() doc reference

Commit a29c0e4b2e86 ("memcg-v1: remove memcg move locking code") removed
folio_memcg_lock().  Delete the final lingering documentation reference.

Link: https://lkml.kernel.org/r/20260101225552.3423108-1-gthelen@google.com
Fixes: a29c0e4b2e86 ("memcg-v1: remove memcg move locking code")
Signed-off-by: Greg Thelen <gthelen@google.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: SeongJae Park <sj@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Michal Koutný" <mkoutny@suse.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>