I get the point now. How about this:

diff --git a/lisp/apropos.el b/lisp/apropos.el
index 3c1a413..a2bfee9 100644
--- a/lisp/apropos.el
+++ b/lisp/apropos.el
@@ -505,7 +505,8 @@ apropos-user-option
 ;;;###autoload
 (defun apropos-variable (pattern &optional do-not-all)
   "Show variables that match PATTERN.
-When DO-NOT-ALL is non-nil, show user options only, i.e. behave
+With the optional argument DO-NOT-ALL non-nil (or when called
interactively with
+the prefix \\[universal-argument]), show user options only, i.e. behave
 like `apropos-user-option'."
   (interactive (list (apropos-read-pattern
       (if current-prefix-arg "user option" "variable"))