Add some info to HELP text (no code change otherwise)
Add some info to HELP text (no code change otherwise)

NTP background isn't going to be widely known. So provide a bit of info so that it's done properly if someone cares.   

For example, it's easy to think a small number is enough, as one might with alternative DNS servers, despite NTP website recommending 4-5 or a pool.  So state the consequence of too few servers.

Also we allow entry of a leap second file but no real info about it or even where it comes from (to get an authoritative file or look further ahead).  As this is an "advanced" and hidden option, it's safe to add a bit of explanation to help the user. If it can be made shorter, please do, though.
NOYB/pfsense,NewEraCracker/pfsense,NOYB/pfsense,NewEraCracker/pfsense,phil-davis/pfsense,pfsense/pfsense,pfsense/pfsense,dennypage/pfsense,pfsense/pfsense,NewEraCracker/pfsense,ptorsten/pfsense,phil-davis/pfsense,NewEraCracker/pfsense,pfsense/pfsense,dennypage/pfsense,phil-davis/pfsense,dennypage/pfsense,ptorsten/pfsense,dennypage/pfsense,phil-davis/pfsense,ptorsten/pfsense,NOYB/pfsense