Fix some docstrings (#1902)

* helm.el (helm-display-function): Do it.
(helm-split-window-default-fn):    Do it.
(helm-display-buffer):             Do it.

helm.el

@@ -392,7 +392,7 @@ the default has changed now to avoid flickering."
 (defcustom helm-display-function 'helm-default-display-buffer
   "Function to display *helm* buffer.
 By default, it is `helm-default-display-buffer', which affects
-`helm-full-frame'."
+`helm-full-frame' among others."
   :group 'helm
   :type 'symbol)
 
@@ -2479,6 +2479,14 @@ frame configuration as per `helm-save-configuration-functions'."
                  (select-frame-set-input-focus frame))))))
 
 (defun helm-split-window-default-fn (window)
+  "Default function to split windows before displaying `helm-buffer'.
+
+It is used as default value for
+`helm-split-window-preferred-function' which is then the let-bounded
+value of `split-window-preferred-function' in `helm-display-buffer'.
+When `helm-display-function' which default to
+`helm-default-display-buffer' is called from `helm-display-buffer' the
+value of `split-window-preferred-function' will be used by `display-buffer'."
   (let (split-width-threshold)
     (if (and (fboundp 'window-in-direction)
              ;; Don't try to split when starting in a minibuffer
@@ -2519,7 +2527,10 @@ frame configuration as per `helm-save-configuration-functions'."
 ;;
 (defun helm-display-buffer (buffer)
   "Display BUFFER.
-The function to display `helm-buffer'."
+
+The function used to display `helm-buffer' by calling
+`helm-display-function' which split window with
+`helm-split-window-preferred-function'."
   (let (pop-up-frames
         (split-window-preferred-function
          helm-split-window-preferred-function)