UBIFS: document dark_wm and dead_wm better

Just add more commentaries. Also some commentary fixes for
lprops flags.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

diff --cc fs/ubifs/gc.c
Simple merge

diff --cc fs/ubifs/super.c
Simple merge

diff --cc fs/ubifs/ubifs.h
Simple merge