Update fs.

external/fs/fs.h

@@ -267,9 +267,14 @@ be saved:
 
 ```c
 fs_file_open(pFS, "config/game.cfg", FS_WRITE, &pFile); // Prefixed with "config", so will use the "config" mount point.
-fs_file_open(pFs, "saves/save0.sav", FS_WRITE, &pFile); // Prefixed with "saves", so will use the "saves" mount point.
+fs_file_open(pFS, "saves/save0.sav", FS_WRITE, &pFile); // Prefixed with "saves", so will use the "saves" mount point.
 ```
 
+When opening a file for writing, if you pass in NULL for the `pFS` parameter it will open the file
+like normal using the standard file system. That is it'll work exactly as if you were using stdio
+`fopen()` and you will not be able use mount points. Keep in mind that there is no notion of a
+"current directory" in this library so you'll be stuck with the initial working directory.
+
 By default, you can move outside the mount point with ".." segments. If you want to disable this
 functionality, you can use the `FS_NO_ABOVE_ROOT_NAVIGATION` flag: