pgsql: Update release notes to mention need for ALTER EXTENSION UPDATE.

Update release notes to mention need for ALTER EXTENSION UPDATE.

Maybe we ought to make pg_upgrade do this for you, but it won't happen
in 9.6, so call out the need for it as a migration consideration.

Branch
------
REL9_6_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/7bdf63ccdc70e7168740bdc426195b3b5d508f29

Modified Files
--------------
doc/src/sgml/release-9.6.sgml | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)

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