patch 8.0.0687: minor issues related to quickfix

Problem:    Minor issues related to quickfix.
Solution:   Set the proper return status for all cases in setqflist() and at
            test cases for this.  Move the "adding" flag outside of
            FEAT_WINDOWS. Minor update to the setqflist() help text. (Yegappan
            Lakshmanan)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index c17512a..03f59a9 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -7008,7 +7008,8 @@
 		    title	quickfix list title text
 		Unsupported keys in {what} are ignored.
 		If the "nr" item is not present, then the current quickfix list
-		is modified.
+		is modified. When creating a new quickfix list, "nr" can be
+		set to a value one greater than the quickfix stack size.
 
 		Examples: >
 			:call setqflist([], 'r', {'title': 'My search'})