Fix comment

Summary: It is true. Also, add a useful message about which thread this Future will complete in.

Test Plan: it should still compile

Reviewed By: davejwatson@fb.com

Subscribers: trunkagent, exa, folly-diffs@, jsedgwick, yfeldblum

FB internal diff: D1819849

Tasks: 5705286, 6008465

Signature: t1:1819849:1423074012:79bf11c877eb2f061cb5d07f7ab45fa4cc4a894b

folly/futures/Future.h

@@ -527,8 +527,10 @@ Future<void> via(Executor* executor);
   The Futures are moved in, so your copies are invalid. If you need to
   chain further from these Futures, use the variant with an output iterator.
 
-  XXX is this still true?
-  This function is thread-safe for Futures running on different threads.
+  This function is thread-safe for Futures running on different threads. But
+  if you are doing anything non-trivial after, you will probably want to
+  follow with `via(executor)` because it will complete in whichever thread the
+  last Future completes in.
 
   The return type for Future<T> input is a Future<std::vector<Try<T>>>
   */