argv[] may not be UTF-8 (fix #811)

docs/content/3.manual/manual.yml

@@ -2477,7 +2477,8 @@ sections:
         body: |
 
           Returns the name of the file whose input is currently being
-          filtered.
+          filtered.  Note that this will not work well unless jq is
+          running in a UTF-8 locale.
 
       - title: "`input_line_number`"
         body: |