caddy-frontend: Explain custom_domain a bit better

Users are often providing wrong values which are rejected later. So give the
clear information to the user.

software/caddy-frontend/instance-slave-caddy-input-schema.json

@@ -2,7 +2,7 @@
   "$schema": "http://json-schema.org/draft-04/schema",
   "properties": {
     "custom_domain": {
-      "description": "Custom Domain to use for the website",
+      "description": "Custom Domain to use for the website. Shall contain only letters, numbers and -, and can look like example.com, first2.example.com special-site.example.com.",
       "pattern": "^([a-zA-Z0-9]([a-zA-Z0-9\\-]{0,61}[a-zA-Z0-9])?\\.)+[a-zA-Z]{2,6}$",
       "title": "Custom Domain",
       "type": "string"