pgbench doc fix

pgbench doc (and some comments in pgbench.c) regarding "-M prepared"
option is not quite correct.

------------------------------------------------------------------------
-M querymode
--protocol=querymode

    Protocol to use for submitting queries to the server:

        simple: use simple query protocol.

        extended: use extended query protocol.

        prepared: use extended query protocol with prepared statements.
------------------------------------------------------------------------

Actually "extended" mode uses prepared statements too. The only
difference is, in extended mode *unnamed* prepared statements are
used, while in prepared mode *named* prepared statements are used.

Also, in extended query protocol, prepared statements are always used
anyway. Thus "use extended query protocol with prepared statements"
does not give any useful information to users.

I think this should be changed to:

        prepared: use extended query protocol with named prepared statements.

Patch attached.

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp