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

doc/src/sgml/config.sgml

@@ -317,7 +317,7 @@ UPDATE pg_settings SET setting = reset_val WHERE name = 'configuration_parameter
 
    </sect2>
 
-   <sect2>
+   <sect2 id="config-setting-shell">
     <title>Parameter Interaction via the Shell</title>
 
      <para>

doc/src/sgml/ref/pgbench.sgml

@@ -2887,6 +2887,22 @@ statement latencies in milliseconds, failures and retries:
   </para>
  </refsect2>
 
+ <refsect2>
+  <title>Table Access Methods</title>
+
+  <para>
+   You may specify the <link linkend="tableam">Table Access Method</link>
+   for the pgbench tables.  The environment variable <envar>PGOPTIONS</envar>
+   specifies database configuration options that are passed to PostgreSQL via
+   the command line (See <xref linkend="config-setting-shell"/>).
+   For example, a hypothetical default Table Access Method for the tables that
+   pgbench creates called <literal>wuzza</literal> can be specified with:
+<programlisting>
+PGOPTIONS='-c default_table_access_method=wuzza'
+</programlisting>
+  </para>
+ </refsect2>
+
  <refsect2>
   <title>Good Practices</title>