Add a note to the related section

See https://discourse.gohugo.io/t/hugo-0-27-released/8325/10

content/content-management/related.md

@@ -57,6 +57,10 @@ type PageGenealogist interface {
 ## Configure Related Content
 Hugo provides a sensible default configuration of Related Content, but you can fine-tune this in your configuration, on the global or language level if needed.
 
+{{% note %}}
+If you add a `related` config section, you need to add a complete configuration. It is not possible to just set, say, `includeNewer` and use the rest  from the Hugo defaults.
+{{% /note %}}
+
 Below is a sample `config.toml` section:
 
 ```