tcp: document TCP Fast Open (RFC 7413) in tcp(4)
Adds documentation for the TCP_FASTOPEN socket option
and related MIB variables to the tcp(4) manual page.
PR: 257907
Reviewed by: gbe
Differential Revision: https://reviews.freebsd.org/D31764
(cherry picked from commit 71611b0c688568d513c665e1af3d95fcd50605fa)