Docs/admin-guide/damon/reclaim: document 'skip_anon' parameter

Document the newly added 'skip_anon' parameter of DAMON_RECLAIM, which can
be used to avoid anonymous pages reclamation.

Link: https://lkml.kernel.org/r/20221205230830.144349-5-sj@kernel.org


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