Remove misleading parts of is_site_local() documentation

In case of IPv6Address and IPv6Network is_private is very much not an
equivalent or a replacement of is_site_local().

IPv6 is_private considers many ranges other than RFC 4193's fc00::/7 to
be private (see _IPv6Constants._private_networks for the complete list).

IPv6Address.is_private goes even further and delegates to
IPv4Address.is_private in case of an IPv4-mapped IPv6 address.

Author: jstasiak

Doc/library/ipaddress.rst

@@ -320,9 +320,7 @@ write code that handles both IP versions correctly.  Address objects are
    .. attribute:: is_site_local
 
       ``True`` if the address is reserved for site-local usage.  Note that
-      the site-local address space has been deprecated by :RFC:`3879`. Use
-      :attr:`~IPv4Address.is_private` to test if this address is in the
-      space of unique local addresses as defined by :RFC:`4193`.
+      the site-local address space has been deprecated by :RFC:`3879`.
 
    .. attribute:: ipv4_mapped
 

Lib/ipaddress.py

@@ -2037,8 +2037,6 @@ def is_site_local(self):
         """Test if the address is reserved for site-local.
 
         Note that the site-local address space has been deprecated by RFC 3879.
-        Use is_private to test if this address is in the space of unique local
-        addresses as defined by RFC 4193.
 
         Returns:
             A boolean, True if the address is reserved per RFC 3513 2.5.6.
@@ -2293,8 +2291,6 @@ def is_site_local(self):
         """Test if the address is reserved for site-local.
 
         Note that the site-local address space has been deprecated by RFC 3879.
-        Use is_private to test if this address is in the space of unique local
-        addresses as defined by RFC 4193.
 
         Returns:
             A boolean, True if the address is reserved per RFC 3513 2.5.6.