docs: admin-guide: cgroup-v1: update description of inactive_file

MADV_FREE pages have been moved into the LRU_INACTIVE_FILE list by commit
f7ad2a6c ("mm: move MADV_FREE pages into LRU_INACTIVE_FILE list").

Link: https://lkml.kernel.org/r/20221111034639.3593380-1-wenjian1@xiaomi.com


Signed-off-by: Jian Wen <wenjian1@xiaomi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>