gtask: Clarify when GTask:completed is suitable to use

It’s not suitable to use to check if your own code has already called
`g_task_return_*()`, as it doesn’t directly correlate to that.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

gio/gtask.c

@@ -2454,9 +2454,18 @@ g_task_class_init (GTaskClass *klass)
    * GTask:completed:
    *
    * Whether the task has completed, meaning its callback (if set) has been
-   * invoked. This can only happen after g_task_return_pointer(),
+   * invoked.
+   *
+   * This can only happen after g_task_return_pointer(),
    * g_task_return_error() or one of the other return functions have been called
-   * on the task.
+   * on the task. However, it is not guaranteed to happen immediately after
+   * those functions are called, as the task’s callback may need to be scheduled
+   * to run in a different thread.
+   *
+   * That means it is **not safe** to use this property to track whether a
+   * return function has been called on the #GTask. Callers must do that
+   * tracking themselves, typically by linking the lifetime of the #GTask to the
+   * control flow of their code.
    *
    * This property is guaranteed to change from %FALSE to %TRUE exactly once.
    *