gcancellable: Clarify that GSources hold references to GCancellables

Clarify in the documentation that a GSource created with
g_cancellable_source_new() must be explicitly removed from its
GMainContext before the GCancellable can be finalised.

This could be a common way of leaking GCancellables.

https://bugzilla.gnome.org/show_bug.cgi?id=737259

gio/gcancellable.c

@@ -722,6 +722,8 @@ static GSourceFuncs cancellable_source_funcs =
  * For convenience, you can call this with a %NULL #GCancellable,
  * in which case the source will never trigger.
  *
+ * The new #GSource will hold a reference to the #GCancellable.
+ *
  * Returns: (transfer full): the new #GSource.
  *
  * Since: 2.28