OpenSSL: Update KTLS documentation

KTLS support has been changed to be off by default, and configuration is
via a single "option" rather two "modes". Documentation is updated
accordingly.

Reviewed by: jkim
Obtained from: OpenSSL (6878f4300213cfd7d4f01e26a8b97f70344da100)
MFC after: 5 days
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31441