UPSTREAM: mm: multi-gen LRU: section for memcg LRU

Move memcg LRU code into a dedicated section.  Improve the design doc to
outline its architecture.

Link: https://lkml.kernel.org/r/20230118001827.1040870-5-talumbau@google.com


Change-Id: Id252e420cff7a858acb098cf2b3642da5c40f602
Signed-off-by: T.J. Alumbaugh <talumbau@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 36c7b4db)
Bug: 274865848
Signed-off-by: T.J. Mercier <tjmercier@google.com>