core: Add some gtk-doc
core: Add some gtk-doc

Just documenting a few functions to get in the habit of things.

gatispaeglis/ostree,sujitfulse/ostree,resin-io/ostree,leandrosansilva/ostree,sujitfulse/ostree,resin-io/ostree,gatispaeglis/ostree,sujitfulse/ostree,sujitfulse/ostree,leandrosansilva/ostree,leandrosansilva/ostree,resin-io/ostree,leandrosansilva/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,sujitfulse/ostree,GNOME/ostree,GNOME/ostree,gatispaeglis/ostree,resin-io/ostree,resin-io/ostree,leandrosansilva/ostree,gatispaeglis/ostree,gatispaeglis/ostree