doc: document opening hidden files on Windows

PR-URL: #15409
Fixes: #14553
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Nikolai Vavilov <vvnicholas@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/api/fs.md

@@ -102,6 +102,11 @@ example `fs.readdirSync('c:\\')` can potentially return a different result than
 `fs.readdirSync('c:')`. For more information, see
 [this MSDN page][MSDN-Rel-Path].
 
+*Note:* On Windows, opening an existing hidden file using the `w` flag (either
+through `fs.open` or `fs.writeFile`) will fail with `EPERM`. Existing hidden
+files can be opened for writing with the `r+` flag. A call to `fs.ftruncate` can
+be used to reset the file contents.
+
 ## Threadpool Usage
 
 Note that all file system APIs except `fs.FSWatcher()` and those that are