pgsql: Document that the HTML documentation must be built before the PDF

Log Message:
-----------
Document that the HTML documentation must be built before the PDF
documentation.

Modified Files:
--------------
pgsql/doc/src/sgml:
docguide.sgml (r1.78 -> r1.79)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/docguide.sgml?r1=1.78&r2=1.79)