Clarify fs.symlink() method description

The original wording gives the impression that fs.symlink() only works on windows. It has been updated to make it clear that the `type` argument (not the whole function) can only be used on windows.

**BEFORE:**

`type` argument can be either `'dir'`, `'file'`, or `'junction'` (default is `'file'`).  It is only used on Windows (ignored on other platforms).

**AFTER:**

The `type` argument can be set to `'dir'`, `'file'`, or `'junction'` (default is `'file'`) and is only available on Windows (ignored on other platforms).