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.