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