Add clarification around URL mappings and file targets.

We received !191781 reporting issues with URL mappings set to a file
failing to resolve due to the activity proxy adding a trailing slash to
the resolved URL.

We don't consider this a supported use case, and have added a note
indicating that this may lead to unexpected behavior.

Author: jkap

docs/activities/Development_Guides.mdx

@@ -220,6 +220,7 @@ To add or modify your application's URL mappings, click on `Activities -> URL Ma
   | PREFIX                | TARGET                   |
   |-----------------------|--------------------------|
   | `/google/{subdomain}` | `{subdomain}.google.com` |
+- Targets must point to a directory; setting a target to a file (e.g. `example.com/index.html`) is unsupported and may lead to unexpected behavior.
 - Because of how URL globbing works, if you have multiple prefix urls with the same initial path, you must place the shortest of the prefix paths last in order for each url mapping to be reachable. For example, if you have `/foo` and `/foo/bar`, you must place the url `/foo/bar` before `/foo` or else the mapping for `/foo/bar` will never be reached.
 
 | ✅ DO                                                 | ❌ DON'T                                                    |