Change aggregated log format of pgbench.

Change aggregated log format of pgbench.

Commit 4a39f87acd changed the aggregated log format. Problem is, now
the explanatory paragraph for the log line in the document is too
long. Also the log format included more optional columns, and it's
harder to parse the log lines.  This commit tries to solve the
problems.

- There's no optional log columns anymore. If a column is not
  meaningful with provided pgbench option, it will be presented as 0.

- Reorder the log columns so that it's easier to parse them.

- Adjust explanatory paragraph for the log line in the doc.

Discussion: https://postgr.es/m/flat/202203280757.3tu4ovs3petm%40alvherre.pgsql