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/quickref.txt b/runtime/doc/quickref.txt
index b309091..68b2eee 100644
--- a/runtime/doc/quickref.txt
+++ b/runtime/doc/quickref.txt
@@ -910,6 +910,7 @@
 'smartcase'	  'scs'     no ignore case when pattern has uppercase
 'smartindent'	  'si'	    smart autoindenting for C programs
 'smarttab'	  'sta'     use 'shiftwidth' when inserting <Tab>
+'smoothscroll'	  'sms'     scroll by screen lines when 'wrap' is set
 'softtabstop'	  'sts'     number of spaces that <Tab> uses while editing
 'spell'			    enable spell checking
 'spellcapcheck'   'spc'     pattern to locate end of a sentence