tcp: vnetify sysctl variables ack_war_timewindow and ack_war_cnt

As suggested by glebius@. While there, improve the documentation.

Reviewed by: Peter Lei, cc
Sponsored by: Netflix, Inc
Differential Revision: https://reviews.freebsd.org/D46140

(cherry picked from commit 4036380e029708f5d6ad7aa599ce1bba9d7c067b)