patch 9.0.0445: when opening/closing window text moves up/down

Problem:    When opening/closing window text moves up/down.
Solution:   Add the 'splitscroll' option.  When off text will keep its
            position as much as possible.
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index b60abc2..336d449 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -7499,6 +7499,18 @@
 	When on, splitting a window will put the new window right of the
 	current one. |:vsplit|
 
+			*'splitscroll'* *'spsc'* *'nosplitscroll'* *'nospsc'*
+'splitscroll' 'spsc'	boolean	(default on)
+			global
+	The value of this option determines the scroll behavior when opening,
+	closing or resizing horizontal splits. When "on", splitting a window
+	horizontally will keep the same relative cursor position in the old and
+	new window, as well windows that are resized. When "off", scrolling
+	will be avoided to stabilize the window content. Instead, the cursor
+	position will be changed when necessary. In this case, the jumplist
+	will be populated with the previous cursor position. Scrolling cannot
+	be guaranteed to be avoided when 'wrap' is enabled.
+
 			   *'startofline'* *'sol'* *'nostartofline'* *'nosol'*
 'startofline' 'sol'	boolean	(default on)
 			global