pgsql: Document how to identify an invalid index after a failed CREATE

Log Message:
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Document how to identify an invalid index after a failed CREATE INDEX
CONCURRENTLY.

Gregory Stark

Modified Files:
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pgsql/doc/src/sgml/ref:
create_index.sgml (r1.60 -> r1.61)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_index.sgml.diff?r1=1.60&r2=1.61)