pgsql: Some small docs improvements motivated by reading the comments

Log Message:
-----------
Some small docs improvements motivated by reading the comments for the
7.4 interactive docs.

Modified Files:
--------------
pgsql/doc/src/sgml:
ddl.sgml (r1.35 -> r1.36)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ddl.sgml.diff?r1=1.35&r2=1.36)
history.sgml (r1.23 -> r1.24)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/history.sgml.diff?r1=1.23&r2=1.24)
query.sgml (r1.41 -> r1.42)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/query.sgml.diff?r1=1.41&r2=1.42)
start.sgml (r1.37 -> r1.38)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/start.sgml.diff?r1=1.37&r2=1.38)