pgsql: Add a note about how to check for bare < and & in SGML docs,

Log Message:
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Add a note about how to check for bare < and & in SGML docs, before the
knowledge disappears again.

Modified Files:
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pgsql/doc/src/sgml:
Makefile (r1.107 -> r1.108)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/Makefile?r1=1.107&amp;r2=1.108)