clarified error code 19

docs/curl.1

@@ -827,7 +827,8 @@ FTP couldn't set binary. Couldn't change transfer method to binary.
 .IP 18
 Partial file. Only a part of the file was transfered.
 .IP 19
-FTP couldn't RETR file. The RETR command failed.
+FTP couldn't download/access the given file, the RETR (or similar) command
+failed.
 .IP 20
 FTP write error. The transfer was reported bad by the server.
 .IP 21