[937-doc-should-mention-rfc8357-relay-port-as-supported] Added RFC 8357 support in doc

doc/sphinx/arm/dhcp4-srv.rst

@@ -5661,6 +5661,12 @@ The following standards are currently supported:
    6842 <https://tools.ietf.org/html/rfc6842>`__: Server by default sends
    back client-id option. That capability may be disabled. See :ref:`dhcp4-echo-client-id` for details.
 
+-  *Generalized UDP Source Port for DHCP Relay*, `RFC 8357
+   <https://tools.ietf.org/html/rfc8357>`__: The Kea server is able
+   to handle Relay Agent Information Source Port suboption in a received
+   message, remembers the UDP port  and sends back reply to the same relay
+   agent using this UDP port.
+
 .. _dhcp4-limit:
 
 DHCPv4 Server Limitations

doc/sphinx/arm/dhcp6-srv.rst

@@ -5722,6 +5722,12 @@ The following standards are currently supported:
    options indicated in this specification are supported by the DHCPv6
    server.
 
+-  *Generalized UDP Source Port for DHCP Relay*, `RFC 8357
+   <https://tools.ietf.org/html/rfc8357>`__: The Kea server is able
+   to handle Relay Source Port option in a received Relay-Forward
+   message, remembers the UDP port and sends back Relay-Reply with a
+   copy of the option to the relay agent using this UDP port.
+
 -  *Dynamic Host Configuration Protocol for IPv6 (DHCPv6)*, `RFC 8415
    <https://tools.ietf.org/html/rfc8415>`__: New DHCPv6 protocol specification
    which obsoletes RFC 3315, RFC 3633, RFC 3736, RFC 4242, RFC 7083, RFC 7283,