doc: note readStream.path if fd is specified

it'll be `undefined` if `fd` is specified.

Refs: nodejs#40013

PR-URL: nodejs#40252
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/fs.md

@@ -5807,7 +5807,8 @@ added: v0.1.93
 The path to the file the stream is reading from as specified in the first
 argument to `fs.createReadStream()`. If `path` is passed as a string, then
 `readStream.path` will be a string. If `path` is passed as a {Buffer}, then
-`readStream.path` will be a {Buffer}.
+`readStream.path` will be a {Buffer}. If `fd` is specified, then
+`readStream.path` will be `undefined`.
 
 #### `readStream.pending`
 <!-- YAML