Note that ntpd_sync_on_start is a preferred alternative to ntpdate_enable.

A similar note is already present in the description of the
ntpd_sync_on_start variable.

This patch adds a note to the description of the ntpdate_enable variable.
This way it would be easier to spot. Otherwise a user might skip the part
of the manual describing ntpd_sync_on_start if they stop reading after
learning about ntpdate_enable.

Reviewed by: bcr
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D16519