Log Message:
-----------
Add note pointing out that read-only commands no longer consume
command IDs.
Modified Files:
--------------
pgsql/doc/src/sgml:
ddl.sgml (r1.80 -> r1.81)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ddl.sgml?r1=1.80&r2=1.81)