repo: Document ostree_repo_sign_commit(), and add introspection data
repo: Document ostree_repo_sign_commit(), and add introspection data

The important bit for introspection is (allow-none) on the homedir.

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