gio: Add various missing property documentation comments

Previously these properties would have been documented using the strings
from the pspec, but those will be removed in the following commit. Re-add
the documentation using those strings, but as gi-docgen documentation
comments.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #2991

gio/gsocketconnection.c

@@ -445,6 +445,13 @@ g_socket_connection_class_init (GSocketConnectionClass *klass)
   stream_class->close_async = g_socket_connection_close_async;
   stream_class->close_finish = g_socket_connection_close_finish;
 
+  /**
+   * GSocketConnection:socket:
+   *
+   * The underlying [class@Gio.Socket].
+   *
+   * Since: 2.22
+   */
   g_object_class_install_property (gobject_class,
                                    PROP_SOCKET,
                                    g_param_spec_object ("socket",