Improve TransactionIdDidAbort() documentation.

Improve TransactionIdDidAbort() documentation.

Document that TransactionIdDidAbort() won't indicate that transactions
that were in-progress during a crash have aborted.  Tie this to existing
discussion of the TransactionIdDidCommit() and TransactionIdDidCommit()
protocol that code in heapam_visibility.c (and a few other places) must
observe.

Follow-up to bugfix commit eb5ad4ff.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAH2-Wzn4bEEqgmaUQL3aJ73yM9gAeK-wE4ngi7kjRjLztb+P0w@mail.gmail.com