doc: improve pipe description

I spent hours trying to make it work only to
learn that it's not supported in node anymore,
so I thought I'd save other people the time in the future.

https://groups.google.com/g/nodejs/c/SxNKLclbM5k?pli=1

PR-URL: #42295
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>

doc/api/child_process.md

@@ -784,6 +784,9 @@ pipes between the parent and child. The value is one of the following:
    `child_process` object as [`subprocess.stdio[fd]`][`subprocess.stdio`]. Pipes
    created for fds 0, 1, and 2 are also available as [`subprocess.stdin`][],
    [`subprocess.stdout`][] and [`subprocess.stderr`][], respectively.
+   Currently, these are not actual Unix pipes and therefore the child process
+   can not use them by their descriptor files,
+   e.g. `/dev/fd/2` or `/dev/stdout`.
 2. `'overlapped'`: Same as `'pipe'` except that the `FILE_FLAG_OVERLAPPED` flag
    is set on the handle. This is necessary for overlapped I/O on the child
    process's stdio handles. See the