In the emacs-25 branch, I see the doc-string for apropos-variable as:

"Show variables that match PATTERN.
When DO-NOT-ALL is non-nil, show user options only, i.e. behave
like `apropos-user-option'."

How about the below patch:

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).

--
Kaushal Modi