pgsql: Improve formatting of PQexecParams argument descriptions, per

Log Message:
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Improve formatting of PQexecParams argument descriptions, per
Theo Kramer.

Modified Files:
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pgsql/doc/src/sgml:
libpq.sgml (r1.219 -> r1.220)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml.diff?r1=1.219&r2=1.220)