Mention which delimiter is accepted in cli args with multiple values

Currently help doesn't mention any delimiter (but in fact delimiter is a comma):

        --peer <[HOST][:PORT]>
            A list of network addresses of other instances. Used during cluster initialization and
            joining an instance to an existing cluster.

            Defaults to `--advertise` value which results in creating a new cluster

            [env: PICODATA_PEER=]

Also correct docs