pgsql: Add some useful cross-references to the CREATE EXTENSION referen

Add some useful cross-references to the CREATE EXTENSION reference page.

Per a suggestion from Josh Kupershmidt, though I modified his patch
quite a lot.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/dd95a1d3dd52cb9848d54ab9962156f666c4ca39

Modified Files
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doc/src/sgml/ref/create_extension.sgml | 32 ++++++++++++++++++++++++++------
1 files changed, 26 insertions(+), 6 deletions(-)