mm: multi-gen LRU: section for rmap/PT walk feedback

Add a section for lru_gen_look_around() in the code and the design doc.

Link: https://lkml.kernel.org/r/20230118001827.1040870-3-talumbau@google.com


Signed-off-by: T.J. Alumbaugh <talumbau@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>