gcancellable: Clarify that GSources hold references to GCancellables
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

endlessm/glib,MathieuDuponchelle/glib,cention-sany/glib,tamaskenez/glib,krichter722/glib,mzabaluev/glib,tchakabam/glib,mzabaluev/glib,MathieuDuponchelle/glib,gale320/glib,tamaskenez/glib,ieei/glib,krichter722/glib,tchakabam/glib,MathieuDuponchelle/glib,endlessm/glib,gale320/glib,ieei/glib,cention-sany/glib,tchakabam/glib,johne53/MB3Glib,johne53/MB3Glib,tamaskenez/glib,tchakabam/glib,gale320/glib,lukasz-skalski/glib,tamaskenez/glib,ieei/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,krichter722/glib,cention-sany/glib,MathieuDuponchelle/glib,endlessm/glib,johne53/MB3Glib,MathieuDuponchelle/glib,Distrotech/glib,krichter722/glib,lukasz-skalski/glib,mzabaluev/glib,endlessm/glib,cention-sany/glib,lukasz-skalski/glib,Distrotech/glib,mzabaluev/glib,tamaskenez/glib,ieei/glib,mzabaluev/glib,johne53/MB3Glib,Distrotech/glib,ieei/glib,Distrotech/glib,krichter722/glib,gale320/glib,lukasz-skalski/glib,lukasz-skalski/glib,cention-sany/glib,Distrotech/glib,gale320/glib,tchakabam/glib