Improve storage/filesystem docs

add hints about how to use the `fileext` and `fileignoreext` parameters.

Author: bernhardreiter

docs/config.rst

@@ -422,13 +422,15 @@ Local
     :param fileext: The file extension to use (e.g. ``.txt``). Contained in the
         href, so if you change the file extension after a sync, this will
         trigger a re-download of everything (but *should* not cause data-loss
-        of any kind).
+        of any kind). To be compatible with the ``vset to the empty string`` format you have
+        to either use ``.vcf`` or ``.ics``. Note that metasync won't work
+        if you use an empty string here.
     :param encoding: File encoding for items, both content and filename.
     :param post_hook: A command to call for each item creation and
         modification. The command will be called with the path of the
         new/updated file.
-    :param fileeignoreext: The file extention to ignore,
-        the default is ``.tmp``.
+    :param fileeignoreext: The file extention to ignore. It is only useful
+        if fileext is set to the empty string. The default is ``.tmp``.
 
 .. storage:: singlefile