Document asymmetric SEE pruning trick

Here are the tests:

sprt @ 60+0.05
ELO: 3.53 +-2.8 (95%) LOS: 99.3%
Total: 18794 W: 3098 L: 2907 D: 12789

16000 @ 60+0.05
ELO: 1.39 +-3.1 (95%) LOS: 81.0%
Total: 16000 W: 2689 L: 2625 D: 10686

16000 @ 15+0.05
ELO: 2.82 +-3.3 (95%) LOS: 95.1%
Total: 16000 W: 3148 L: 3018 D: 9834

No functional change

Signature: 4969307

src/search.cpp

@@ -1225,7 +1225,8 @@ split_point_start: // At split points actual search starts from here
               continue;
           }
 
-          // Prune moves with negative or equal SEE
+          // Prune moves with negative or equal SEE.
+          // Also prune moves with positive SEE where capturing loses a tempo and SEE < beta - futilityBase.
           if (   futilityBase < beta
               && depth < DEPTH_ZERO
               && pos.see_asymm(move, beta - futilityBase) <= 0)