let fibers::Semaphore be documented as FIFO

Summary: Its implementation wakes waiters in FIFO order and this property was unlikely ever to be changed. By Hyrum's Law, this property is now required. So let it be documented.

Differential Revision: D48069483

fbshipit-source-id: 67f6a71aa0c13513c49a72d9add036dc528f308b

folly/fibers/Semaphore.h

@@ -30,6 +30,10 @@ namespace fibers {
 /*
  * Fiber-compatible semaphore. Will safely block fibers that wait when no
  * tokens are available and wake fibers when signalled.
+ *
+ * Fair. Waiters are awoken in FIFO order. (Note: whether the callers see FIFO
+ * order depends on the executors, wrapping async types, and the existence of
+ * happens-before relationships between async wait operations.)
  */
 class Semaphore {
  public: