Replace :xref-backend setting with :jump

Which takes :definitions, :references, :documentation and :xref-backend.
Each accepts a function that will jump to definitions, references, and
documentation respectively.

If a major-mode has an :xref-backend, you don't need :definitions or
:references.

modules/lang/emacs-lisp/config.el

@@ -7,6 +7,7 @@
   :config
   (set! :repl 'emacs-lisp-mode #'+emacs-lisp/repl)
   (set! :eval 'emacs-lisp-mode #'+emacs-lisp-eval)
+  (set! :jump 'emacs-lisp-mode :documentation #'describe-symbol)
   (set! :rotate 'emacs-lisp-mode
         :symbols '(("t" "nil")
                    ("let" "let*")