pgsql: Add an "incompatibility" entry to 9.1 release notes about CREATE

Add an "incompatibility" entry to 9.1 release notes about CREATE EXTENSION.

We've now seen more than one gripe from somebody who didn't get the memo
about how to install contrib modules in 9.1.  Try to make it a little more
prominent that you aren't supposed to call the scripts directly anymore.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/ae92eff6b93c6ee18ca5c48958c355f5a448f8f5

Modified Files
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doc/src/sgml/release-9.1.sgml |   18 ++++++++++++++++++
1 files changed, 18 insertions(+), 0 deletions(-)