pgsql: Add comment on why SGML -wfully-tagged flag is needed (for older

Log Message:
-----------
Add comment on why SGML -wfully-tagged flag is needed (for older SGML
tool chains).

Modified Files:
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pgsql/doc/src/sgml:
Makefile (r1.98 -> r1.99)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/Makefile?r1=1.98&r2=1.99)