trafgen: update help description

After commit f9591eebe40c (trafgen: lexer/parser: fix cpu() selection
and whitespacing), we also need to fixup the help description, so that
users start using the cpu(X-Y) notation instead. Also, reintroduce the
alias csum16 for csumip.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>