pgsql: Add some index entries for RAISE and exception handling in

Log Message:
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Add some index entries for RAISE and exception handling in PL/PgSQL. Per
suggestion from Rainer Brandt.

Modified Files:
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pgsql/doc/src/sgml:
plpgsql.sgml (r1.59 -> r1.60)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.59&r2=1.60)