Explain pruning pgstats accounting subtleties.

Explain pruning pgstats accounting subtleties.

Add a comment explaining why the pgstats accounting used during
opportunistic heap pruning operations (to maintain the current number of
dead tuples in the relation) needs to compensate by subtracting away the
number of new LP_DEAD items.  This is needed so it can avoid completely
forgetting about tuples that become LP_DEAD items during pruning -- they
should still count.

It seems more natural to discuss this issue at the only relevant call
site (opportunistic pruning), since the same issue does not apply to the
only other caller (the VACUUM call site).  Move everything there too.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-Wzm7f+A6ej650gi_ifTgbhsadVW5cujAL3punpupHff5Yg@mail.gmail.com