doc: explain use of json_populate_record{set}()

The set-returning nature of these functions make their use unclear. The
modified paragraph was added in PG 9.4.

Reported-by:  <yshaladi@denodo.com>

Discussion:  https://postgr.es/m/152571684246.9460.18059951267371255159@wrigleys.postgresql.org

Backpatch-through: 9.4