Clearly describe API of callback in code (#7279)

* Clearly describe API of callback in code

* fix syntax error

Author: kylemh

packages/react-router/docs/api/Prompt.md

@@ -23,11 +23,15 @@ Will be called with the next `location` and `action` the user is attempting to n
 
 ```jsx
 <Prompt
-  message={location =>
-    location.pathname.startsWith("/app")
+  message={(location, action) => {
+    if (action === 'POP') {
+      console.log("Backing up...")
+    }
+
+    return location.pathname.startsWith("/app")
       ? true
       : `Are you sure you want to go to ${location.pathname}?`
-  }
+  }}
 />
 ```