rpc.tlsservd: Add the -2 option to the man page

Since the KTLS now supports TLS1.3, the daemons default
to version 1.3, since the draft (to be an RFC someday)
requires TLS1.3.

However, since FreeBSD 13,0, 13,1 uses TLS1.2 for
NFS-over-TLS, the "-2" option is added to both daemons
for compatibility with FreeBSD 13.0, 13.1.

This patch updates the man pages for this.

This is a content change.

(cherry picked from commit e2c72fecfc51d376600b29dfea737a3d1054e34a)