pgsql: 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(at)bowt(dot)ie>
Reviewed-By: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/CAH2-Wzn4bEEqgmaUQL3aJ73yM9gAeK-wE4ngi7kjRjLztb+P0w@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/af3855cb77b94a0321b77d7d9a9849700ea6a758

Modified Files
--------------
src/backend/access/heap/heapam_visibility.c | 33 ++++++++++++++++-------------
src/backend/access/transam/transam.c | 9 +++++++-
2 files changed, 26 insertions(+), 16 deletions(-)