org-eldoc: Fix compatibility with eldoc 1.11 and Emacs 27

* contrib/lisp/org-eldoc.el (org-eldoc-documentation-function): Check
  before invoking elisp eldoc functions from Emacs 28.

The previous check assumed that the presence of eldoc 1.11 bindings
implied elisp-mode bindings that come with Emacs>=28, but eldoc 1.11
is available on GNU Elpa so the assumption doesn't always hold.

TINYCHANGE

contrib/lisp/org-eldoc.el

@@ -144,7 +144,9 @@
      (cond ((or
              (string= lang "emacs-lisp")
              (string= lang "elisp"))
-	    (cond ((boundp 'eldoc-documentation-functions) ; Emacs>=28
+	    (cond ((and (boundp 'eldoc-documentation-functions) ; Emacs>=28
+			(fboundp 'elisp-eldoc-var-docstring)
+			(fboundp 'elisp-eldoc-funcall))
 		   (let ((eldoc-documentation-functions
 			  '(elisp-eldoc-var-docstring elisp-eldoc-funcall)))
 		     (eldoc-print-current-symbol-info)))