Documentation fix on pgbench \aset command

Hi,

The current documentation of pgbench’s \aset command states:

“If a query returns no row, no assignment is made and the variable
 can be tested for existence to detect this.”

However, this is inaccurate, since variable existence check (like \if :{?var})
is not yet supported, although such a feature was proposed in [1] and discussed
again in [2], where \aset itself was introduced.

Therefore, I’ve attached a patch to remove this part from the documentation.

[1] https://www.postgresql.org/message-id/flat/alpine.DEB.2.20.1802191909500.21372%40lancre
[2] https://www.postgresql.org/message-id/flat/alpine.DEB.2.21.1904081914200.2529%40lancre

Best regards,
Yugo Nagata
-- 
Yugo Nagata <nagata@sraoss.co.jp>