Give permanent-local property to per-buffer variables without defaults.

Give permanent-local property to per-buffer variables without defaults.

* doc/emacs/custom.texi (Examining): Update C-h v message.

* lisp/bindings.el: Assign a non-nil permanent-local property to
per-buffer variables which lack a default value.

* lisp/help-fns.el (describe-variable): In the "automatically becomes
local" notice, take note of permanent-local variables.

Fixes: debbugs:11930