Better document how mate scores are stored in TT

Better document how mate scores are stored in TT

During the search we score a mate as "plies to mate
from the root" to compare in an homogeneous way the
values returned by different sub-trees. However we
store in TT a mate score as "plies to mate from the
current position" the let the TT value remain valid
across the game.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>