pgbench: Clarify documentation for \gset and \aset.

pgbench: Clarify documentation for \gset and \aset.

This commit updates the pgbench documentation to list \gset and \aset
as separate terms for easier reading. It also clarifies that \gset raises
an error if the query returns zero or multiple rows, and explains how to
detect cases where the query with \aset returned no rows.

Author: Yugo Nagata <[email protected]>
Reviewed-by: Fujii Masao <[email protected]>
Discussion: https://postgr.es/m/20250626180125.5b896902a3d0bcd93f86c240@sraoss.co.jp