Adding PRR sysctls to tcp(4) man page

Documenting the newly added, and enabled by default,
Proportional Rate Reduction algorithm's governing sysctls.

Reviewed by: kbowling, rgrimes
MFC after: 3 days
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D28568

(cherry picked from commit a78fee8182614593992e841fbb0d4a958f70da0f)