Refclock #46, GPSD_JSON doc does not match the code.

The docs/driver46.txt file makes no mention of the preferred operational mode of thhe driver. This mode has no proper name, but is partially documented in ntpd/refclock_gpsdjson.c on line 36. Until we define a good name for it we can call it tandem mode.

Tandem mode exposes both the NMEA and PPS times to the user. See bug #57 (closed) for configuration and output examples.

The ntpd.org documentation notes that the refclock mode word is broken. I suspect this is true and the NTPsec doc for #46 (closed) should reflect this brokeness, if true. If in fact the NTPsec refclock mode word does work it should be documented as a user visible change from ntpd.org.

[edited to fix typo #48 (closed) -> #46 (closed)]