gsource: document priority of child sources
gsource: document priority of child sources

Add a note to the documentation that child sources cannot have their priority
changed independently from their parent.  Add a g_return_if_fail() to the
public API in order to enforce this.

This was already a reality due to the check in
g_source_set_priority_unlocked(), but it was never explicitly documented.

https://bugzilla.gnome.org/show_bug.cgi?id=724706

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