discovery: clarify why the caching of children is valid

Yuya Nishihara pointed out that the code looks wrong without this clarification.
(And, unsurprisingly, Yuya is right)

mercurial/setdiscovery.py

@@ -175,6 +175,10 b' class partialdiscovery(object):'
     def _childrengetter(self):
 
         if self._childrenmap is not None:
+            # During discovery, the `undecided` set keep shrinking.
+            # Therefore, the map computed for an iteration N will be
+            # valid for iteration N+1. Instead of computing the same
+            # data over and over we cached it the first time.
             return self._childrenmap.__getitem__
 
         # _updatesample() essentially does interaction over revisions to look