Updated documentation for new sequence binding

I have marged Tom's description of the new sequence binding with text I
was working on.  I modified it to follow the existing "we used to do X,
now we do Y" pattern in the surrounding entries:

    http://candle.pha.pa.us/tmp/pgsql/release.html#RELEASE-8-1

I also added an SQL query that will show and update a pre-8.1 loaded
schema to the new oid-based sequence storage use.

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  Bruce Momjian                        |  http://candle.pha.pa.us
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