Fix the docs on Future callbacks' ExecutionContext

They said the callback ran either immediately or in the
provided ExecutionContext, but really the callback is
guaranteed to be run in the ExecutionContext.
However ExecutionContext.execute() may be run either
immediately or asynchronously from some other ExecutionContext.

src/library/scala/concurrent/Future.scala

@@ -84,8 +84,13 @@ import language.higherKinds
  *  }}}
  *
  * @define callbackInContext
- * The provided callback may run immediately (if the future has been completed)
- * and otherwise runs in the provided implicit `ExecutionContext`.
+ * The provided callback always runs in the provided implicit
+ *`ExecutionContext`, though there is no guarantee that the
+ * `execute()` method on the `ExecutionContext` will be called once
+ * per callback or that `execute()` will be called in the current
+ * thread. That is, the implementation may run multiple callbacks
+ * in a batch within a single `execute()` and it may run
+ * `execute()` either immediately or asynchronously.
  */
 trait Future[+T] extends Awaitable[T] {