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_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/4cf2115072b5bdc31748872cfc6ba0c32ba36f6f

Modified Files
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doc/src/sgml/extend.sgml               |    6 ++++++
doc/src/sgml/ref/create_extension.sgml |    6 ++++++
2 files changed, 12 insertions(+), 0 deletions(-)