Fix documentation of argument type of json_agg and jsonb_agg

Fix documentation of argument type of json_agg and jsonb_agg

json_agg was originally designed to aggregate records. However, it soon
became clear that it is useful for aggregating all kinds of values and
that's what we have on 9.3 and 9.4, and in head for it and jsonb_agg.
The documentation suggested otherwise, so this fixes it.