add mention about IPv6 former site-local address block.

Author: Matthieu Lienart

Doc/library/ipaddress.rst

@@ -243,7 +243,8 @@ write code that handles both IP versions correctly.  Address objects are
       ``True`` if the address is noted as reserved by the IETF.
       For IPv4, this is only ``240.0.0.0/4``, the ``Reserved`` address block.
       For IPv6, this is all addresses `allocated <iana-ipv6-address-space_>`__ as
-      ``Reserved by IETF`` for future use.
+      ``Reserved by IETF`` for future use, except ``fec0::/10`` a former Site-Local scoped
+      address prefix (see :attr:`~IPv6Address.is_site_local`).
 
       Note: ``is_reserved`` is not related to the IPv4 address block value of the
       ``Reserved-by-Protocol`` column in iana-ipv4-special-registry_.