Doc: enhance query cache documents.

Mention that we can use PGPOOL SET CACHE DELETE or
pcp_invalidate_query_cache command to delete query cache.

Author: tatsuo-ishii

doc.ja/src/sgml/memcache.sgml

@@ -39,7 +39,10 @@
     /*NO QUERY CACHE*/SELECT now();
    </programlisting>
    Because it just prevents to create a cache entry for the query, and
-   does not affect the query using the FORCE QUERY CACHE comment.
+   does not affect the query using the FORCE QUERY CACHE comment.  You
+   can delete such a query cache by
+   using <xref linkend="SQL-PGPOOL-SET-CACHE"> command,
+   or <xref linkend="PCP-INVALIDATE-QUERY-CACHE"> command.
    </programlisting>
    Because it just prevents to create a cache entry for the query, and
    does not affect the query using the FORCE QUERY CACHE comment.
@@ -60,6 +63,7 @@
     /*NO QUERY CACHE*/SELECT now();
    </programlisting>
    なぜなら、これはこのクエリのキャッシュエントリが作成されないようにするだけで、FORCE QUERY CACHEコメントを使ったクエリには影響を与えないからです。
+   このようなクエリキャッシュは、<xref linkend="SQL-PGPOOL-SET-CACHE">コマンドか、<xref linkend="PCP-INVALIDATE-QUERY-CACHE">コマンドで削除できます。
    </para>
   <para>
   <!--

doc/src/sgml/memcache.sgml

@@ -39,7 +39,10 @@
     /*NO QUERY CACHE*/SELECT now();
    </programlisting>
    Because it just prevents to create a cache entry for the query, and
-   does not affect the query using the FORCE QUERY CACHE comment.
+   does not affect the query using the FORCE QUERY CACHE comment.  You
+   can delete such a query cache by
+   using <xref linkend="SQL-PGPOOL-SET-CACHE"> command,
+   or <xref linkend="PCP-INVALIDATE-QUERY-CACHE"> command.
   </para>
   <para>
    Note that for following query: