In documentaiton, clarify which commands have reduced WAL volume for

wal_level = minimum.

Backpatch to 9.1 and 9.0.

doc/src/sgml/config.sgml

@@ -1451,11 +1451,18 @@ SET ENABLE_SEQSCAN TO OFF;
         This parameter can only be set at server start.
        </para>
        <para>
-        In <literal>minimal</> level, WAL-logging of some bulk operations, like
-        <command>CREATE INDEX</>, <command>CLUSTER</> and <command>COPY</> on
-        a table that was created or truncated in the same transaction can be
-        safely skipped, which can make those operations much faster (see
-        <xref linkend="populate-pitr">). But minimal WAL does not contain
+        In <literal>minimal</> level, WAL-logging of some bulk
+        operations can be safely skipped, which can make those
+        operations much faster (see <xref linkend="populate-pitr">).
+        Operations in which this optimization can be applied include:
+        <simplelist>
+         <item>CREATE TABLE AS</item>
+         <item>CREATE INDEX</item>
+         <item>CLUSTER</item>
+         <item>COPY into tables that were created or truncated in the same
+         transaction
+        </simplelist>
+        But minimal WAL does not contain
         enough information to reconstruct the data from a base backup and the
         WAL logs, so either <literal>archive</> or <literal>hot_standby</>
         level must be used to enable