WelcomeMessage: specify that the user has to be local.

docs/config.md

@@ -98,7 +98,7 @@ config :pleroma, Pleroma.Mailer,
 * `autofollowed_nicknames`: Set to nicknames of (local) users that every new user should automatically follow.
 * `no_attachment_links`: Set to true to disable automatically adding attachment link text to statuses
 * `welcome_message`: A message that will be send to a newly registered users as a direct message.
-* `welcome_user_nickname`: The nickname of the user that sends the welcome message.
+* `welcome_user_nickname`: The nickname of the local user that sends the welcome message.
 
 ## :logger
 * `backends`: `:console` is used to send logs to stdout, `{ExSyslogger, :ex_syslogger}` to log to syslog