Doc: minor documentation improvements about extensions.

Document the formerly-undocumented behavior that schema and comment
control-file entries for an extension are honored only during initial
installation, whereas other properties are also honored during updates.

While at it, do some copy-editing on the recently-added docs for CREATE
EXTENSION ... CASCADE, use links for some formerly vague cross references,
and make a couple other minor improvements.

Back-patch to 9.6 where CASCADE was added.  The other parts of this
could go further back, but they're probably not important enough to
bother.

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