Real-time feature config is under admin area

It looks like at some point the `Application settings` name was changed
to use `Admin area -> Settings` in navigation.

This doc about altering `Real-time features` (polling) still references
the older area name, and required an update.

Also added a link to the `Admin Area` documentation to allow for easier
discovery.

doc/administration/polling.md

@@ -9,7 +9,8 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 The GitLab UI polls for updates for different resources (issue notes, issue
 titles, pipeline statuses, etc.) on a schedule appropriate to the resource.
 
-In "Application settings -> Real-time features" you can configure "Polling
+In **[Admin Area](../user/admin_area/index.md) > Settings > Preferences > Real-time features**,
+you can configure "Polling
 interval multiplier". This multiplier is applied to all resources at once,
 and decimal values are supported. For the sake of the examples below, we will
 say that issue notes poll every 2 seconds, and issue titles poll every 5