gio: Make minor docs improvements

This commit changes a comment in _g_dbus_worker_do_read_cb() to be
slightly more useful. At least in my experience debugging an
intermittent unit test failure in another project, this failure
condition occurred because although g_test_dbus_down() ensures that the
session GDBusConnection has exit-on-close set to FALSE before killing
its dbus-daemon, there was still a GDBusConnection on the system bus
which hit this failed read code path, because we had
DBUS_SYSTEM_BUS_ADDRESS set to the address of the #GTestDBus daemon, to
appease libudisks.

Also, make a few other minor improvements to the docs.

gio/gtestdbus.c

@@ -801,7 +801,7 @@ g_test_dbus_stop (GTestDBus *self)
  * Stop the session bus started by g_test_dbus_up().
  *
  * This will wait for the singleton returned by g_bus_get() or g_bus_get_sync()
- * is destroyed. This is done to ensure that the next unit test won't get a
+ * to be destroyed. This is done to ensure that the next unit test won't get a
  * leaked singleton from this test.
  */
 void