doc: appendFileSync accepts fd as well

Refs: #6508
PR-URL: #6707
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/fs.md

@@ -327,7 +327,7 @@ _Note: Specified file descriptors will not be closed automatically._
 
 ## fs.appendFileSync(file, data[, options])
 
-* `file` {String | Buffer}
+* `file` {String | Buffer | Number} filename or file descriptor
 * `data` {String | Buffer}
 * `options` {Object | String}
   * `encoding` {String | Null} default = `'utf8'`