pgsql: Doc: minor documentation improvements about extensions.

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.

Branch
------
REL9_6_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/01280ee203b7c34b48e61f874b497951cc63d8a7

Modified Files
--------------
doc/src/sgml/extend.sgml | 32 ++++++++++++--------
doc/src/sgml/ref/create_extension.sgml | 54 ++++++++++++++--------------------
2 files changed, 42 insertions(+), 44 deletions(-)