Update 1.7.6.md

Author: japgolly

doc/changelog/1.7.6.md

@@ -5,6 +5,10 @@
 * Add `.timeout(timeLimit)` to `AsyncCallback` instances so that you can limit the amount of time to wait for the async
   procedure to complete.
 
+  *(Note: there's no built-in concept of cancellation here.
+  If your procedure doesn't finish in time, this will return `None` when the time limit is reached however, the
+  underlying procedure will become orphaned and continue to run in the background until complete.)*
+
 * Add convenience methods `CallbackTo` for getting the current time:
   * `now: CallbackTo[Instant]`
   * `currentTimeMillis: CallbackTo[Long]`