feat(alerting): change placeholder to info-popover

public/app/features/alerting/partials/notification_edit.html

@@ -59,16 +59,20 @@
 
     <div class="gf-form-group" ng-if="ctrl.model.type === 'slack'">
       <h3 class="page-heading">Slack settings</h3>
-      <div class="gf-form">
+      <div class="gf-form max-width-30">
         <span class="gf-form-label width-6">Url</span>
         <input type="text" required class="gf-form-input max-width-30" ng-model="ctrl.model.settings.url" placeholder="Slack incoming webhook url"></input>
       </div>
-      <div class="gf-form">
+      <div class="gf-form max-width-30">
         <span class="gf-form-label width-6">Recipient</span>
-        <input type="text" class="gf-form-input max-width-30"
+        <input type="text"
+          class="gf-form-input max-width-30"
           ng-model="ctrl.model.settings.recipient"
-          placeholder="Override default channel or user, use #channel-name or @username">
+          data-placement="right">
         </input>
+        <info-popover mode="right-absolute">
+          Override default channel or user, use #channel-name or &lt;@username&gt;&lt;/@username&gt;
+        </info-popover>
       </div>
     </div>