koofr: use lib/encoder

Co-authored-by: Nick Craig-Wood <nick@craig-wood.com>

docs/content/koofr.md

@@ -86,6 +86,18 @@ To copy a local directory to an Koofr directory called backup
 
     rclone copy /home/source remote:backup
 
+#### Restricted filename characters
+
+In addition to the [default restricted characters set](/overview/#restricted-characters)
+the following characters are also replaced:
+
+| Character | Value | Replacement |
+| --------- |:-----:|:-----------:|
+| \         | 0x5C  | ＼           |
+
+Invalid UTF-8 bytes will also be [replaced](/overview/#invalid-utf8),
+as they can't be used in XML strings.
+
 <!--- autogenerated options start - DO NOT EDIT, instead edit fs.RegInfo in backend/koofr/koofr.go then run make backenddocs -->
 ### Standard Options