pgsql: Update bitmap index description: < Such indexes could be more

Log Message:
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Update bitmap index description:

< Such indexes could be more compact if there are only a few unique values.

Such indexes could be more compact if there are only a few distinct values.
Such indexes can also be compressed. Keeping such indexes updated can be
costly.

Modified Files:
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pgsql/doc:
TODO (r1.1392 -> r1.1393)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1392&amp;r2=1.1393)