pgsql: More updates and copy-editing.

Log Message:
-----------
More updates and copy-editing.  Rearrange order of sections a little bit
to put more widely useful info before less widely useful info.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        extend.sgml (r1.28 -> r1.29)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/extend.sgml.diff?r1=1.28&r2=1.29)
        postgres.sgml (r1.71 -> r1.72)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/postgres.sgml.diff?r1=1.71&r2=1.72)
        rules.sgml (r1.36 -> r1.37)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/rules.sgml.diff?r1=1.36&r2=1.37)
        trigger.sgml (r1.38 -> r1.39)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/trigger.sgml.diff?r1=1.38&r2=1.39)
        xfunc.sgml (r1.90 -> r1.91)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xfunc.sgml.diff?r1=1.90&r2=1.91)
        xplang.sgml (r1.26 -> r1.27)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xplang.sgml.diff?r1=1.26&r2=1.27)