Do not set `trusted-content` in major modes

Do not set `trusted-content` in major modes

* lisp/progmodes/elisp-mode.el (lisp-interaction-mode):
* lisp/ielm.el (inferior-emacs-lisp-mode): Do not set `trusted-content.
* lisp/ielm.el (ielm):
* lisp/simple.el (get-scratch-buffer-create): Set `trusted-content` here
instead.
* lisp/files.el (trusted-content): Doc fix; warn against setting this
option to :all in a major or mode mode.
Problem reported by Max Nikulin <manikulin@gmail.com>.

(cherry picked from commit 5485ea6aef91c65a0ce300347db3c0ac138ad550)