list_lru: expand list_lru_add() docs with info about sublists

list_lru: expand list_lru_add() docs with info about sublists

The documentation for list_lru_add() and list_lru_del() has not been
updated since lru lists were originally introduced by commit a38e40824844
("list: add a new LRU list type").  Back then, list_lru stored all of the
items in a single list, but the implementation has since been expanded to
use many sublists internally.

Thus, update the docs to mention that the requirements about not using the
item with several lists at the same time also applies not using different
sublists.  Also mention that list_lru items are reparented when the memcg
is deleted as discussed on the LKML [1].

Also fix incorrect use of 'Return value:' which should be 'Return:'.

Link: https://lore.kernel.org/all/Z0eXrllVhRI9Ag5b@dread.disaster.area/
Link: https://lkml.kernel.org/r/20241129-list_lru_memcg_docs-v2-1-e285ff1c481b@google.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Acked-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>