pgsql: Document how to find TeX overflow boxes in our documentation

Log Message:
-----------
Document how to find TeX overflow boxes in our documentation build, per
details from Alvaro Herrera.

Modified Files:
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pgsql/doc/src/sgml:
docguide.sgml (r1.81 -> r1.82)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/docguide.sgml?r1=1.81&r2=1.82)