docs: add note about fs.rmdir()

fs.rmdir() on the file (not directory) results in different errors on
Windows to everything else

Fixes: https://github.com/nodejs/node/issues/8797
PR-URL: https://github.com/nodejs/node/pull/14323
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

doc/api/fs.md

@@ -2004,6 +2004,9 @@ changes:
 Asynchronous rmdir(2). No arguments other than a possible exception are given
 to the completion callback.
 
+*Note*: Using `fs.rmdir()` on a file (not a directory) results in an `ENOENT`
+error on Windows and an `ENOTDIR` error on POSIX.
+
 ## fs.rmdirSync(path)
 <!-- YAML
 added: v0.1.21
@@ -2018,6 +2021,9 @@ changes:
 
 Synchronous rmdir(2). Returns `undefined`.
 
+*Note*: Using `fs.rmdirSync()` on a file (not a directory) results in an `ENOENT`
+error on Windows and an `ENOTDIR` error on POSIX.
+
 ## fs.stat(path, callback)
 <!-- YAML
 added: v0.0.2