rpc.tlsclntd: 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.

Reviewed by: pauamma_gundo.com
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35290