doc: document fs.watchFile() bigint option

This commit documents the bigint option to fs.watchFile(), which
has been supported since v10.5.0.

PR-URL: #32128
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

doc/api/fs.md

@@ -3820,6 +3820,9 @@ fs.watch('somedir', (eventType, filename) => {
 <!-- YAML
 added: v0.1.31
 changes:
+  - version: v10.5.0
+    pr-url: https://github.com/nodejs/node/pull/20220
+    description: The `bigint` option is now supported.
   - version: v7.6.0
     pr-url: https://github.com/nodejs/node/pull/10739
     description: The `filename` parameter can be a WHATWG `URL` object using
@@ -3828,6 +3831,7 @@ changes:
 
 * `filename` {string|Buffer|URL}
 * `options` {Object}
+  * `bigint` {boolean} **Default:** `false`
   * `persistent` {boolean} **Default:** `true`
   * `interval` {integer} **Default:** `5007`
 * `listener` {Function}
@@ -3853,7 +3857,8 @@ fs.watchFile('message.text', (curr, prev) => {
 });
 ```
 
-These stat objects are instances of `fs.Stat`.
+These stat objects are instances of `fs.Stat`. If the `bigint` option is `true`,
+the numeric values in these objects are specified as `BigInt`s.
 
 To be notified when the file was modified, not just accessed, it is necessary
 to compare `curr.mtime` and `prev.mtime`.