pgsql: Merge some user-submitted suggestions for improvement into the

Log Message:
-----------
Merge some user-submitted suggestions for improvement into the
documentation. Mostly add some <xref>s, fix a few typos, and
document that zlib is required in the installation docs.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        datatype.sgml (r1.161 -> r1.162)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml.diff?r1=1.161&r2=1.162)
        extend.sgml (r1.29 -> r1.30)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/extend.sgml.diff?r1=1.29&r2=1.30)
        installation.sgml (r1.241 -> r1.242)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/installation.sgml.diff?r1=1.241&r2=1.242)
        plpgsql.sgml (r1.77 -> r1.78)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.77&r2=1.78)
        xfunc.sgml (r1.105 -> r1.106)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xfunc.sgml.diff?r1=1.105&r2=1.106)
    pgsql/doc/src/sgml/ref:
        prepare.sgml (r1.15 -> r1.16)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/prepare.sgml.diff?r1=1.15&r2=1.16)