Add comment explaining interface of maxBetweenExclusive

ConflictSet.cpp

@@ -1010,6 +1010,8 @@ downLeftSpine:
   }
 }
 
+// Return the max version among all keys starting with the search path of n +
+// [child], where child in (begin, end)
 int64_t maxBetweenExclusive(Node *n, int begin, int end) {
   assert(-1 <= begin);
   assert(begin <= 256);