gdbusmessage: Improve documentation for g_dbus_message_get_header()
gdbusmessage: Improve documentation for g_dbus_message_get_header()

The caller is responsible for checking the type of the returned
GVariant.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib