Update docs

Signed-off-by: mueller-ma <[email protected]>

docs/USAGE.md

@@ -333,9 +333,8 @@ Create a new "custom namespace" metadata on an Item with the name `link_to_more`
 
 ### UI command Item
 
-Similar to the UI command Item in Main UI you can use an Item to control the Sitemaps.
-
-TODO: Add link to Main UI
+Similar to the [UI command Item in Main UI](https://next.openhab.org/docs/mainui/about.html#ui-command-item) you can use an Item to control the Sitemaps.
+All commands except `popup:` is supported by the app.
 
 ## Multi server support