Sort out ElDoc backward compatibility of eldoc-documentation-function

Sort out ElDoc backward compatibility of eldoc-documentation-function

As explained previously, we can't simply make
eldoc-documentation-function an variable alias for
eldoc-documentation-strategy, because ElDoc is pre-loaded in Emacs <
28, where it holds at least one buffer-local binding.  So if eldoc.el
is loaded in those versions, we do the variable alias binding in
reverse.  We do this using a macro
eldoc--documentation-strategy-defcustom to at load time in which
direction to make the variable alias.

* lisp/emacs-lisp/eldoc.el
(eldoc--documentation-strategy-defcustom): Helper macro.
(eldoc-documentation-strategy, eldoc-documentation-function): Use it.
(Version): Bump to 1.5.0