Docs/admin-guide/damon/sysfs: document 'LRU_PRIO' scheme action

This commit documents the 'lru_prio' scheme action for DAMON sysfs
interface.

Link: https://lkml.kernel.org/r/20220613192301.8817-6-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>