docs: update docs for prefix4/next_node.ip4

docs/user/site.rst

@@ -21,12 +21,14 @@ site_code
     The code of your community. It is good practice to use the TLD of
     your community here.
 
-prefix4
+prefix4 \: optional
     The IPv4 Subnet of your community mesh network in CIDR notation, e.g.
     ::
 
        prefix4 = '10.111.111.0/18'
 
+    Required if ``next_node.ip4`` is set.
+
 prefix6
     The IPv6 subnet of your community mesh network, e.g.
     ::
@@ -147,6 +149,8 @@ next_node \: package
         mac = 'ca:ff:ee:ba:be:00'
       }
 
+    The IPv4 next-node address is optional.
+
 mesh \: optional
     Options specific to routing protocols.