* lisp/emacs-lisp/inline.el (define-inline): Add a doc string.

lisp/emacs-lisp/inline.el

@@ -124,6 +124,10 @@ After VARS is handled, BODY is evaluated in the new environment."
 
 ;;;###autoload
 (defmacro define-inline (name args &rest body)
+  "Define an inline function NAME with arguments ARGS and body in BODY.
+
+This is like `defmacro', but has several advantages.
+See Info node `(elisp)Defining Functions' for more details."
   ;; FIXME: How can this work with CL arglists?
   (declare (indent defun) (debug defun) (doc-string 3))
   (let ((doc (if (stringp (car-safe body)) (list (pop body))))