Improve Gio::Socket documentation.

* gio/src/socket.hg: Added documentation for the bind(),
  listen() and accept() methods, based on original Glib
  API reference but mentioning our exceptions rather than
  GError or return values.
  Bug #725178
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.
Gio::Socket::bind() docs: Use @a instead of <tt></tt> for a parameter.

* gio/src/socket.hg: @a is the doxygen way to say that we are
  referring to a parameter in the method being documented.