domain migrate: Add text that subdomains are not supported

webadmin/src/views/certs.html

@@ -12,6 +12,7 @@
                         <div class="form-group" ng-class="{ 'has-error': dnsCredentialsForm.customDomainId.$invalid }" uib-tooltip="{{ config.provider === 'caas' ? '' : 'Changing the domain is not yet supported' }}">
                             <label class="control-label" for="customDomainId">Domain name</label>
                             <input type="text" class="form-control" ng-model="dnsCredentials.customDomain" id="customDomainId" name="customDomainId" ng-disabled="dnsCredentials.busy || config.provider !== 'caas'" check-tld placeholder="example.com" required autofocus>
+                            <p>&nbsp;<span ng-show="dnsCredentialsForm.customDomainId.$error.invalidSubdomain" class="text-danger">Subdomains are <a href="https://cloudron.io/references/selfhosting.html#domain-setup" target="_blank" title="Domain documentation">not supported</a></span></p>
                         </div>
 
                         <div class="form-group">