patch 9.0.0640: cannot scroll by screen line if a line wraps

Problem:    Cannot scroll by screen line if a line wraps.
Solution:   Add the 'smoothscroll' option.  Only works for CTRL-E and CTRL-Y
            so far.
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index aa69dd1..070f8cb 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -7302,6 +7302,14 @@
 	reset.
 	NOTE: This option is reset when 'compatible' is set.
 
+			*'smoothscroll'* *'sms'* *'nosmoothscroll'* *'nosms'*
+'smoothscroll' 'sms'	boolean  (default off)
+			local to window
+	Scrolling works with screen lines.  When 'wrap' is set and the first
+	line in the window wraps part of it may not be visible, as if it is
+	above the window.
+	NOTE: only partly implemented, works with CTRL-E and CTRL-Y.
+
 					*'softtabstop'* *'sts'*
 'softtabstop' 'sts'	number	(default 0)
 			local to buffer