There is a second problem:
There are two variants of the documentation:
a) text in def usage() of /roundup/scripts/roundup_mailgw.py
b) ./share/man/man1/roundup-mailgw.1

b) seems to be more recent.

And a third one: 
the "<instance home>" part is optional as well.

Fixed the --help text with hg4990:63574f1f3570 
and opened Issue2550885 for the "second problem"
and other implications.

Thanks Karl-Phillip!