Add a FIXME on the odd probe_ab()

Given a position probe_ab() does a kind of qsearch,
but instead of evaluating the position at the begin,
through a table look up, it performs a depth-first
search and only at the end checks for current position
score.

src/syzygy/tbprobe.cpp

@@ -1713,7 +1713,7 @@ WDLScore probe_ab(Position& pos, WDLScore alpha, WDLScore beta, int *success)
         }
     }
 
-    value = probe_wdl_table(pos, success);
+    value = probe_wdl_table(pos, success); // FIXME why this is not at the beginning?
 
     if (*success == 0)
         return WDLDraw;