Document How Commit Handles Aborted Transactions

Hi,

The commit reference page lacks an "Outputs" section even though it is
capable of outputting both "COMMIT" and "ROLLBACK".

The attached adds this section, describes when each applies, and then
incorporates the same into the main description for commit as well as the
transaction section of the tutorial - which presently seems to be the main
discussion area for the topic (the Concurrency Control chapter lacks a
section for this introductory material).

This was noted as being needed by Tom Lane back into 2006 but it
never happened.
https://www.postgresql.org/message-id/28798.1142608067%40sss.pgh.pa.us

It came up again when I was answering a question on Slack regarding "commit
and chain" wondering whether the "and chain" could be made conditional
(i.e., could the new transaction start aborted) on whether commit outputted
"commit" or "rollback".

Its left implied that this behavior of "rollback" is standard-conforming.
Please feel free to suggest/add language to the Compatibility section if
this is not the case.

David J.