patch 9.0.0647: the 'splitscroll' option is not a good name

Problem:    The 'splitscroll' option is not a good name.
Solution:   Rename 'splitscroll' to 'splitkeep' and make it a string option,
            also supporting "topline". (Luuk van Baal, closes #11258)
diff --git a/runtime/doc/quickref.txt b/runtime/doc/quickref.txt
index 68b2eee..4d13a47 100644
--- a/runtime/doc/quickref.txt
+++ b/runtime/doc/quickref.txt
@@ -919,8 +919,8 @@
 'spelloptions'	  'spo'     options for spell checking
 'spellsuggest'	  'sps'     method(s) used to suggest spelling corrections
 'splitbelow'	  'sb'	    new window from split is below the current one
+'splitkeep'	  'spk'     determines scroll behavior for split windows
 'splitright'	  'spr'     new window is put right of the current one
-'splitscroll'	  'spsc'    determines scroll behavior for split windows
 'startofline'	  'sol'     commands move cursor to first non-blank in line
 'statusline'	  'stl'     custom format for the status line
 'suffixes'	  'su'	    suffixes that are ignored with multiple match