Get rid of the per-column documentation of underlying functions, which did
far more to clutter the view descriptions than it did to be helpful, and
was rather incomplete and typo-ridden anyway.  Instead suggest that people
consult the definitions of the standard views to see the underlying
functions.

The older functions for obtaining individual facts about backends are now
somewhat obsoleted by pg_stat_get_activity, which means that they are not
documented by any standard view.  So I put that information into a separate
table.  (Maybe we should just deprecate them instead?)

In passing, fix a couple more documentation errors.