The following is what I would do (the specific wording is irrelevant).
--- window.el 6 Jun 2008 08:00:59 -0000 1.137
+++ window.el 7 Jun 2008 12:40:44 -0000
@@ -743,9 +743,12 @@
(defcustom split-window-preferred-function nil
"How `display-buffer' shall split windows.
-Choices are `Vertically', `Horizontally', and `Sensibly' where
-the latter attempts to split wide windows horizontally, narrow
-ones vertically. Alternatively, you can set this to a function
-called with a window as single argument to split that window in
-two and return the new window."
+Choices are:
+ - `horizontally' windows are split horizontally
+ - `sensibly' attempt to split wide windows horizontally,
+ narrow ones vertically
+ - nil windows are split vertically
+ - a function it will be called with a window as a single
+ argument; it should split it in two and
+ return the new window."
:type '(choice
(const :tag "Vertically" nil)