doc: cluster: bump stability level to 'unstable'

As discussed on the mailing list: the module will not go away but the
API will continue to receive updates as the need arises.

Link: https://groups.google.com/forum/#!topic/nodejs/uqyTcQfimAI
Message-ID: <7384b30e-b64c-4086-b78f-b5acca9842a9@googlegroups.com>