Fix Eldoc problem when loading on Emacs 26.3

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