tcp: cleanup of nits after use of accessor tcp_get_flags

Remove unneeded th_x2 initalization, use named constants
instead of magic numbers (fixing one oversight) and add
some line breaks. Expand one man page slightly.

No functional change intended.

Reviewed By: tuexen, cc
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D48065