doc: update position type to integer | null in fs

PR-URL: #57745
Reviewed-By: LiviaMedeiros 
Reviewed-By: James M Snell 

Author: y-hsgw

doc/api/fs.md

@@ -740,7 +740,7 @@ added:
 * `options` {Object}
   * `offset` {integer} **Default:** `0`
   * `length` {integer} **Default:** `buffer.byteLength - offset`
-  * `position` {integer} **Default:** `null`
+  * `position` {integer|null} **Default:** `null`
 * Returns: {Promise}
 
 Write `buffer` to the file.
@@ -4984,7 +4984,7 @@ added:
 * `options` {Object}
   * `offset` {integer} **Default:** `0`
   * `length` {integer} **Default:** `buffer.byteLength - offset`
-  * `position` {integer} **Default:** `null`
+  * `position` {integer|null} **Default:** `null`
 * `callback` {Function}
   * `err` {Error}
   * `bytesWritten` {integer}
@@ -6515,7 +6515,7 @@ added:
 * `options` {Object}
   * `offset` {integer} **Default:** `0`
   * `length` {integer} **Default:** `buffer.byteLength - offset`
-  * `position` {integer} **Default:** `null`
+  * `position` {integer|null} **Default:** `null`
 * Returns: {number} The number of bytes written.
 
 For detailed information, see the documentation of the asynchronous version of