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
* 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