Fix Eldoc problem when loading on Emacs 26.3

When defining the obsolete variable alias for old
eldoc-documentation-function (which now points to the newer
eldoc-documentation-strategy), one gets the error "don't know how to
make a localized vareiable an alias".  I'm not sure, but I suspect
this is because Eldoc is preloaded in Emacs 26.3 and the
eldoc-documentation-function variable is already set locally by some
Elisp buffer.

Uninterning the symbol shortly before defining the alias seems to fix
it.

* lisp/emacs-lisp/eldoc.el (eldoc-documentation-function):
Unintern on load.
(Version): Bump to 1.3.0

lisp/emacs-lisp/eldoc.el

@@ -5,7 +5,7 @@
 ;; Author: Noah Friedman <friedman@splode.com>
 ;; Keywords: extensions
 ;; Created: 1995-10-06
-;; Version: 1.2.0
+;; Version: 1.3.0
 ;; Package-Requires: ((emacs "26.3"))
 
 ;; This is a GNU ELPA :core package.  Avoid functionality that is not
@@ -535,6 +535,9 @@ Meant as a value for `eldoc-documentation-strategy'."
                         (if (stringp str) (funcall callback str))
                         nil))))
 
+;; FIXME: The unintern fixes "don't know how to make a localized
+;; variable an alias".
+(unintern 'eldoc-documentation-function)
 (define-obsolete-variable-alias 'eldoc-documentation-function
   'eldoc-documentation-strategy "eldoc-1.1.0")