This makes a few changes:
 - It merges the two mentions of gpsd (in the SHM docs) into one
   section.
 - It removes the talk about ntpd combining serial and PPS data.  gpsd
   already joins these data streams.
 - It no longer describes SHM as "the ancient way to have GPSD talk to
   ntpd".
 - The old note about "minpoll 4" is removed.  It has been replaced by
   portions of text from gpsd-time-service-howto.adoc.

Fixes #668