Gio::Socket docs: Do not say that the Cancellable (reference) can be 0.
Gio::Socket docs: Do not say that the Cancellable (reference) can be 0.

* gio/src/socket.hg: That makes sense only in the C API, which uses
  a pointer. We instead have method overloads without that parameter.

johne53/MB3Glibmm,johne53/MB3Glibmm,johne53/MB3Glibmm,johne53/MB3Glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm