doc: Mention PGDATABASE as supported by pgbench

PGHOST, PGPORT and PGUSER were already mentioned, but not PGDATABASE.
Like 5aaa584f, backpatch down to 12.

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

doc/src/sgml/ref/pgbench.sgml

@@ -151,6 +151,18 @@ pgbench <optional> <replaceable>options</replaceable> </optional> <replaceable>d
 
     <variablelist>
 
+     <varlistentry>
+      <term><replaceable class="parameter">dbname</replaceable></term>
+      <listitem>
+       <para>
+        Specifies the name of the database to test in. If this is
+        not specified, the environment variable
+        <envar>PGDATABASE</envar> is used. If that is not set, the
+        user name specified for the connection is used.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><option>-i</option></term>
       <term><option>--initialize</option></term>
@@ -900,6 +912,7 @@ pgbench <optional> <replaceable>options</replaceable> </optional> <replaceable>d
 
   <variablelist>
    <varlistentry>
+    <term><envar>PGDATABASE</envar></term>
     <term><envar>PGHOST</envar></term>
     <term><envar>PGPORT</envar></term>
     <term><envar>PGUSER</envar></term>