pgsql: Adjust hints and docs to suggest CREATE EXTENSION not CREATELAN

Adjust hints and docs to suggest CREATE EXTENSION not CREATE LANGUAGE.

The core PLs have been extension-ified for seven years now, and we can
reasonably hope that all out-of-core PLs have been too.  So adjust a few
places that were still recommending CREATE LANGUAGE as the user-level
way to install a PL.

Discussion: https://postgr.es/m/CA+TgmoaJTUDMSuSCg4k08Dv8vhbrJq9nP3ZfPbmysVz_616qxw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2e83e6bd74ee8d4d119edf8272406f23e6c546cf

Modified Files
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doc/src/sgml/ref/create_transform.sgml | 4 ++--
doc/src/sgml/ref/do.sgml               | 2 +-
src/backend/commands/functioncmds.c    | 4 ++--
3 files changed, 5 insertions(+), 5 deletions(-)