mm: vmscan: improve comment on low-page cache handling

mm: vmscan: improve comment on low-page cache handling

Fix comment style and elaborate on why anonymous memory is force-scanned
when file cache runs low.

Signed-off-by: Johannes Weiner <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Reviewed-by: Michal Hocko <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Satoru Moriya <[email protected]>
Cc: Simon Jeons <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>