Doc: commit performs rollback of aborted transactions.

Doc: commit performs rollback of aborted transactions.

The COMMIT command handles an aborted transaction in the same
manner as the ROLLBACK command, but this wasn't explained in
its official reference page.  Also mention that behavior in
the tutorial's material on transactions.

Also add a comment mentioning that we don't raise an exception
for COMMIT within an aborted transaction, as the SQL standard
would have us do.

Hyperlink a couple of cross-references while we're at it.

Author: David G. Johnston <david.g.johnston@gmail.com>
Reviewed-by: Gurjeet Singh <gurjeet@singh.im>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAKFQuwYgYR3rWt6vFXw=ZWZ__bv7PqvdOnHujG+UyqE11f+3sg@mail.gmail.com