"Show variables that match PATTERN.
When DO-NOT-ALL is non-nil, show user options only, i.e. behave
diff --git a/lisp/apropos.el b/lisp/apropos.el
index 3c1a413..fdd85b4 100644
--- a/lisp/apropos.el
+++ b/lisp/apropos.el
@@ -505,7 +505,7 @@ 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 prefix argument DO-NOT-ALL non-nil, show user options only, i.e. behave
like `apropos-user-option'."
(interactive (list (apropos-read-pattern
(if current-prefix-arg "user option" "variable"))
I simply added the term "prefix argument" (based on the doc-string of quit-window).