ntp: Improve descriptions in manual pages

+ ntpd added to ntp.conf(5) description (search keywords)
+ expand NTP so these pages are shown when apropos time
+ "standard" => "reference" for increased consistency

• removed redundant or duplicated search keywords

freebsd-src: c7a33fe3 (ntp: Improve descriptions)
ntp.org bug: https://bugs.ntp.org/show_bug.cgi?id=3936
Reviewed by: Harlan Stenn <stenn@nwtime.org>
Reviewed by: Cy Schubert <cy@nwtime.org, cy@FreeBSD.org>