pgsql: doc: Mention PGDATABASE as supported by pgbench

doc: Mention PGDATABASE as supported by pgbench

PGHOST, PGPORT and PGUSER were already mentioned, but not PGDATABASE.
Like 5aaa584, backpatch down to 12.

Reported-by: Christophe Courtois
Discussion: https://postgr.es/m/161399398648.21711.15387267201764682579@...
Backpatch-through: 12

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1f56ae322943e9c9b3be4b2c480891a291861bf7

Modified Files
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doc/src/sgml/ref/pgbench.sgml | 13 +++++++++++++
1 file changed, 13 insertions(+)