Add descriptive comment to FutureAlreadyContinued

Summary:
A comment to `FutureAlreadyContinued` to explain to readers in what circumstance it is thrown.

(Note: this ignores all push blocking failures!)

Reviewed By: yfeldblum

Differential Revision: D8460097

fbshipit-source-id: 49657e030efe72c026c1c3eea3612952746bb0e1

folly/futures/Future.h

@@ -57,6 +57,11 @@ class FOLLY_EXPORT FutureInvalid : public FutureException {
   FutureInvalid() : FutureException("Future invalid") {}
 };
 
+/// At most one continuation may be attached to any given Future.
+///
+/// If a continuation is attached to a future to which another continuation has
+/// already been attached, then an instance of FutureAlreadyContinued will be
+/// thrown instead.
 class FOLLY_EXPORT FutureAlreadyContinued : public FutureException {
  public:
   FutureAlreadyContinued() : FutureException("Future already continued") {}