Try to be more helpful when matrix_homeserver_generic_secret_key appears to be undefined

roles/custom/matrix-base/tasks/validate_config.yml

@@ -25,6 +25,8 @@
       The `matrix_homeserver_generic_secret_key` variable must be defined and have a non-null and non-empty value.
 
       If you're observing this error on a new installation, you should ensure that the `matrix_homeserver_generic_secret_key` is defined.
+      If you think you've defined it, but are still getting this error, then it's likely that you have a typo
+      in your domain name in `inventory/hosts` or in one of the directories leading up to your `inventory/host_vars/matrix.DOMAIN/vars.yml` file.
 
       If you're observing this error on an existing homeserver installation, you can fix it easily and in a backward-compatible way by adding
       `{% raw %}matrix_homeserver_generic_secret_key: "{{ matrix_synapse_macaroon_secret_key }}"{% endraw %}`