pgsql: Document the ability to specify TableAM for pgbench

Document the ability to specify TableAM for pgbench

Upcoming custom Table Access Methods (TableAM) need benchmarking.  Despite
pgbench doesn't have an explicit option for TableAM specification, one can
specify it using PGOPTION environmental variable.  The present commit documents
this way to specify TableAM for pgbench.

Discussion: https://postgr.es/m/CAC77N6ih%3DLbhZQXV76grEsaVQkBL464Y2Foqq9o%3Df4UBfEOfEQ%40mail.gmail.com
Author: Michel Pelletier, Alexander Korotkov
Reviewed-by: Justin Pryzby, Mason Sharp, Michael Paquier

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f77ff083350eb5a2625a4dbfca61d15b66c4918b

Modified Files
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doc/src/sgml/config.sgml      |  2 +-
doc/src/sgml/ref/pgbench.sgml | 16 ++++++++++++++++
2 files changed, 17 insertions(+), 1 deletion(-)