Document behaviour of the `encoding` parameter of fs.writeFile().

Fixes #401.

doc/api/fs.markdown

@@ -361,7 +361,8 @@ returns a buffer.
 ### fs.writeFile(filename, data, encoding='utf8', [callback])
 
 Asynchronously writes data to a file, replacing the file if it already exists.
-`data` can be a string or a buffer.
+`data` can be a string or a buffer. The `encoding` argument is ignored if
+`data` is a buffer.
 
 Example: