doc: add annotation to writeFile data as Object

Fixes: nodejs#39152

doc/api/fs.md

@@ -1280,8 +1280,8 @@ changes:
 -->
 
 * `file` {string|Buffer|URL|FileHandle} filename or `FileHandle`
-* `data` {string|Buffer|TypedArray|DataView|Object|AsyncIterable|Iterable
-  |Stream}
+* `data` {string|Buffer|TypedArray|DataView|AsyncIterable|Iterable|Stream|
+Object}*
 * `options` {Object|string}
   * `encoding` {string|null} **Default:** `'utf8'`
   * `mode` {integer} **Default:** `0o666`
@@ -1290,8 +1290,8 @@ changes:
 * Returns: {Promise} Fulfills with `undefined` upon success.
 
 Asynchronously writes data to a file, replacing the file if it already exists.
-`data` can be a string, a {Buffer}, or an object with an own `toString` function
-property.
+`data` can be a string, a {Buffer}, or, like all variations of `writeFile`, an
+object with an own (not inherited) `toString` function property.
 
 The `encoding` option is ignored if `data` is a buffer.
 
@@ -3930,7 +3930,7 @@ changes:
 -->
 
 * `file` {string|Buffer|URL|integer} filename or file descriptor
-* `data` {string|Buffer|TypedArray|DataView|Object}
+* `data` {string|Buffer|TypedArray|DataView|Object}*
 * `options` {Object|string}
   * `encoding` {string|null} **Default:** `'utf8'`
   * `mode` {integer} **Default:** `0o666`
@@ -3947,7 +3947,9 @@ When `file` is a file descriptor, the behavior is similar to calling
 a file descriptor.
 
 The `encoding` option is ignored if `data` is a buffer.
-If `data` is a normal object, it must have an own `toString` function property.
+
+\* If `data` is a normal object, it must have an own (not inherited) `toString`
+function property.
 
 ```mjs
 import { writeFile } from 'fs';
@@ -5026,7 +5028,7 @@ changes:
 -->
 
 * `file` {string|Buffer|URL|integer} filename or file descriptor
-* `data` {string|Buffer|TypedArray|DataView|Object}
+* `data` {string|Buffer|TypedArray|DataView|Object}*
 * `options` {Object|string}
   * `encoding` {string|null} **Default:** `'utf8'`
   * `mode` {integer} **Default:** `0o666`