extended the description for exit code 9

docs/curl.1

@@ -1217,7 +1217,9 @@ Failed to connect to host.
 .IP 8
 FTP weird server reply. The server sent data curl couldn't parse.
 .IP 9
-FTP access denied. The server denied login.
+FTP access denied. The server denied login or denied access to the particular
+resource or directory you wanted to reach. Most often you tried to change to a
+directory that doesn't exist on the server.
 .IP 10
 FTP user/password incorrect. Either one or both were not accepted by the
 server.