Document the expected input/output of rewriteRequestUrl

Summary:
[`rewriteRequestUrl`](https://metrobundler.dev/docs/configuration/#rewriterequesturl) usually receives paths (eg, from `req.url` when processing a request), but sometimes also absolute URLs (eg, when processing source URL in a symbolicate payload), and the returned value is expected to mirror the input.

This just caught me out, so I'm clarifying in the docs.

We should change this API. It's especially awkward now that Node's `url.parse` is deprecated.

Changelog: Docs

Reviewed By: vzaidman

Differential Revision: D91132265

fbshipit-source-id: 49c2f1af79415ce3a8dcdd099a90c670db2cf721

Author: robhogan

docs/Configuration.md

@@ -643,7 +643,13 @@ The `Middleware` type is an alias for [`connect.HandleFunction`](https://github.
 
 Type: `string => string`
 
-A function that will be called every time Metro processes a URL, after normalization of non-standard query-string delimiters using [`jsc-safe-url`](https://www.npmjs.com/package/jsc-safe-url). Metro will use the return value of this function as if it were the original URL provided by the client. This applies to all incoming HTTP requests (after any custom middleware), as well as bundle URLs in `/symbolicate` request payloads and within the hot reloading protocol.
+A function that will be called every time Metro processes a "URL" (see note), after normalization of non-standard query-string delimiters using [`jsc-safe-url`](https://www.npmjs.com/package/jsc-safe-url). Metro will use the return value of this function as if it were the original URL provided by the client. This applies to all incoming HTTP requests (after any custom middleware), as well as bundle URLs in `/symbolicate` request payloads and within the hot reloading protocol.
+
+:::note
+
+The input may be either an absolute URL (e.g. `https://example.com/foo/bar?baz=qux`) or a path (e.g. `/foo/bar?baz=qux`). The output should use the same form as the input - i.e. the returned value should be an absolute URL if and only if the input is an absolute URL.
+
+:::
 
 #### `forwardClientLogs`