pgsql: Improve documentation about the relationship of extensions and s

Improve documentation about the relationship of extensions and schemas.

There's been some confusion expressed about this point, so clarify.
Extended version of a patch by David Wheeler.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/d189deebd3c9e178c32c420a0df2b1ef9d445119

Modified Files
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doc/src/sgml/extend.sgml               |    6 ++++++
doc/src/sgml/ref/create_extension.sgml |   10 ++++++++--
2 files changed, 14 insertions(+), 2 deletions(-)