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