Reword alpha release note item on SSI.

Per Josh Berkus; some additional explanatory text by me.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/bcf85e3f44a518eb7bb759541189efb6726fe9bc

Modified Files
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doc/src/sgml/release-9.1.sgml |    9 ++++++++-
1 files changed, 8 insertions(+), 1 deletions(-)

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