docs: update comment about prefix4 in site.conf example

docs/site-example/site.conf

@@ -15,7 +15,9 @@
   -- Shorthand of the community.
   site_code = 'ffxx',
 
-  -- Prefixes used within the mesh. Both are required.
+  -- Prefixes used within the mesh.
+  -- prefix6 is required, prefix4 can be omitted if next_node.ip4
+  -- is not set.
   prefix4 = '10.xxx.0.0/20',
   prefix6 = 'fdxx:xxxx:xxxx::/64',