Add a note to the rc.conf(5) manual that ntpd_sync_on_start is a preferred alternative to ntpdate_enable

[0mp]

Summary

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.

[bcr]

OK from manpages.
As this is a content change, you need to bump the .Dd at the top of the man page to the date of the commit. Thanks!

[0mp]

Thanks, @bcr.

@mat, @krion, may I ask you for an approval to commit this?

[0mp]

Thank you!