docs/ftp: improved default value description of port and username options

See #5551

docs/content/sftp.md

@@ -56,9 +56,11 @@ Choose a number from below, or type in your own value
  1 / Connect to example.com
    \ "example.com"
 host> example.com
-SSH username, leave blank for current username, $USER
+SSH username
+Enter a string value. Press Enter for the default ("$USER").
 user> sftpuser
-SSH port, leave blank to use default (22)
+SSH port number
+Enter a signed integer. Press Enter for the default (22).
 port>
 SSH password, leave blank to use ssh-agent.
 y) Yes type in my own password