http://tools.ietf.org/html/rfc5735.html
Special Use IPv4 Addresses

does indeed agree with the docstring

    @property
    def is_unspecified(self):
        """Test if the address is unspecified.

        Returns:
            A boolean, True if this is the unspecified address as defined in
            RFC 5735 3.

        """
        unspecified_address = IPv4Address('0.0.0.0')
        return self == unspecified_address

and makes more sense than http://tools.ietf.org/html/rfc5375.html
IPv6 Unicast Address Assignment Considerations

and so the attached patch will bring them into line.

However it's worth noting that the RFC doesn't say anything about 0.0.0.0 being the 'unspecified' address, (the RFC linked for the IPv6 is_unspecified http://tools.ietf.org/html/rfc2373.html#section-2.5.2 does specifically call it the 'unspecified' address). 5735 3 just says:

3.  Global and Other Specialized Address Blocks

   0.0.0.0/8 - Addresses in this block refer to source hosts on "this"
   network.  Address 0.0.0.0/32 may be used as a source address for this
   host on this network; other addresses within 0.0.0.0/8 may be used to
   refer to specified hosts on this network ([RFC1122], Section
   3.2.1.3).

googling it you can find eg this:

http://en.wikipedia.org/wiki/IPv6_address
says:
 ::/128 — The address with all zero bits is called the unspecified address (corresponding to 0.0.0.0/32 in IPv4).

so perhaps this bit of the docs could be a bit clearer, although I don't know what it should say, perhaps something like:
'Checks if the address is 0.0.0.0 which corresponds to the unspecified address in IPv6'
someone with better networking knowledge than me could say.