doc: document process.std*.fd

Fixes: #28386
Refs: #31292
Refs: nodejs/help#2136

PR-URL: #31395
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/process.md

@@ -2162,6 +2162,14 @@ a [Writable][] stream.
 `process.stderr` differs from other Node.js streams in important ways. See
 [note on process I/O][] for more information.
 
+### `process.stderr.fd`
+
+* {number}
+
+This property refers to the value of underlying file descriptor of
+`process.stderr`. The value is fixed at `2`. In [`Worker`][] threads,
+this field does not exist.
+
 ## `process.stdin`
 
 * {Stream}
@@ -2195,6 +2203,14 @@ In "old" streams mode the `stdin` stream is paused by default, so one
 must call `process.stdin.resume()` to read from it. Note also that calling
 `process.stdin.resume()` itself would switch stream to "old" mode.
 
+### `process.stdin.fd`
+
+* {number}
+
+This property refers to the value of underlying file descriptor of
+`process.stdin`. The value is fixed at `0`. In [`Worker`][] threads,
+this field does not exist.
+
 ## `process.stdout`
 
 * {Stream}
@@ -2213,6 +2229,14 @@ process.stdin.pipe(process.stdout);
 `process.stdout` differs from other Node.js streams in important ways. See
 [note on process I/O][] for more information.
 
+### `process.stdout.fd`
+
+* {number}
+
+This property refers to the value of underlying file descriptor of
+`process.stdout`. The value is fixed at `1`. In [`Worker`][] threads,
+this field does not exist.
+
 ### A note on process I/O
 
 `process.stdout` and `process.stderr` differ from other Node.js streams in