Allow using define-short-documentation-group without loading shortdoc

* lisp/emacs-lisp/shortdoc.el (define-short-documentation-group):
Allow using without loading shortdoc.el (bug#56260).

lisp/emacs-lisp/shortdoc.el

@@ -41,10 +41,12 @@
   '((t :inherit variable-pitch))
   "Face used for a section.")
 
-(defvar shortdoc--groups nil)
+;;;###autoload
+(progn
+  (defvar shortdoc--groups nil)
 
-(defmacro define-short-documentation-group (group &rest functions)
-  "Add GROUP to the list of defined documentation groups.
+  (defmacro define-short-documentation-group (group &rest functions)
+    "Add GROUP to the list of defined documentation groups.
 FUNCTIONS is a list of elements on the form:
 
   (FUNC
@@ -88,8 +90,7 @@ string will be `read' and evaluated.
 
   (FUNC
    :no-eval EXAMPLE-FORM
-   :result RESULT-FORM   ;Use `:result-string' if value is in string form
-   )
+   :result RESULT-FORM)   ;Use `:result-string' if value is in string form
 
 Using `:no-value' is the same as using `:no-eval'.
 
@@ -102,17 +103,16 @@ execution of the documented form depends on some conditions.
 
   (FUNC
    :no-eval EXAMPLE-FORM
-   :eg-result RESULT-FORM ;Use `:eg-result-string' if value is in string form
-   )
+   :eg-result RESULT-FORM) ;Use `:eg-result-string' if value is in string form
 
 A FUNC form can have any number of `:no-eval' (or `:no-value'),
 `:no-eval*', `:result', `:result-string', `:eg-result' and
 `:eg-result-string' properties."
-  (declare (indent defun))
-  `(progn
-     (setq shortdoc--groups (delq (assq ',group shortdoc--groups)
-                                  shortdoc--groups))
-     (push (cons ',group ',functions) shortdoc--groups)))
+    (declare (indent defun))
+    `(progn
+       (setq shortdoc--groups (delq (assq ',group shortdoc--groups)
+                                    shortdoc--groups))
+       (push (cons ',group ',functions) shortdoc--groups))))
 
 (define-short-documentation-group alist
   "Alist Basics"