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>