Note that ntpd_sync_on_start is a preferred alternative to ntpdate_enable.

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