pgsql: Make a quick copy-editing pass over the 9.1 release notes.

Make a quick copy-editing pass over the 9.1 release notes.

Also remove the material about this being an alpha release.

The notes still need a lot of work, but they're more or less presentable
as a beta version now.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/bb1051eb2d5eef060b64788cbec8459c46427fca

Modified Files
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doc/src/sgml/config.sgml | 8 +-
doc/src/sgml/release-9.1.sgml | 291 ++++++++++++++++++-----------------------
2 files changed, 133 insertions(+), 166 deletions(-)