Document the non-effect of TCP-related -p params on UDS in

Upgrading to 6.0.

doc/sphinx/whats-new/upgrading-6.0.rst

@@ -12,7 +12,18 @@ XXX ...
 varnishd parameters
 ===================
 
-XXX: ... terse overview, use refs to link to details ...
+XXX: ...
+
+The parameters :ref:`ref_param_tcp_keepalive_intvl`,
+:ref:`ref_param_tcp_keepalive_probes` and
+:ref:`ref_param_tcp_keepalive_time` are silently ignored for listen
+address that are Unix domain sockets. The parameters
+:ref:`ref_param_accept_filter` and :ref:`ref_param_tcp_fastopen`
+(which your platform may or may not support in the first place) almost
+certainly have no effect on a UDS. It is not an error to use any of
+these parameters with a UDS; you may get error messages in the log for
+``accept_filter`` or ``tcp_fastopen`` (with the VSL tag ``Error`` in
+raw grouping), but they are harmless.
 
 Changes to VCL
 ==============