Add a success indication

Also document the command.

hedgedoc-expire.py

@@ -69,6 +69,7 @@ class EmailSender:
         except Exception as e:
             print(f'Unable to send mail through {self}: {e}')
             raise e
+        print(f'Test email to {self.mail_from} sent successfully.')
 
     def __str__(self):
         return f'EmailSender<{self.hostname},{self.port},{self.username},{self.mail_from}>'
@@ -291,6 +292,7 @@ def main():
         command is one of:
          - check: print a list of revisions and notes to be expired
          - cron: run expire every 24 hours
+         - emailcheck: send a test email from the configured sender to themselves
          - expire: expire old revisions and untouched notes
         
         See https://git.hamburg.ccc.de/CCCHH/hedgedoc-expire