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/src/testdir/test_quickfix.vim b/src/testdir/test_quickfix.vim
index 755ea74..c595458 100644
--- a/src/testdir/test_quickfix.vim
+++ b/src/testdir/test_quickfix.vim
@@ -1720,7 +1720,8 @@
     Xopen
     wincmd p
     call g:Xsetlist([{'filename':'foo', 'lnum':27}])
-    call g:Xsetlist([], 'a', {'title' : 'Sample'})
+    let s = g:Xsetlist([], 'a', {'title' : 'Sample'})
+    call assert_equal(0, s)
     let d = g:Xgetlist({"title":1})
     call assert_equal('Sample', d.title)
 
@@ -1774,7 +1775,8 @@
     endif
 
     " Context related tests
-    call g:Xsetlist([], 'a', {'context':[1,2,3]})
+    let s = g:Xsetlist([], 'a', {'context':[1,2,3]})
+    call assert_equal(0, s)
     call test_garbagecollect_now()
     let d = g:Xgetlist({'context':1})
     call assert_equal([1,2,3], d.context)
@@ -1839,8 +1841,9 @@
     " Test for setting/getting items
     Xexpr ""
     let qfprev = g:Xgetlist({'nr':0})
-    call g:Xsetlist([], ' ', {'title':'Green',
+    let s = g:Xsetlist([], ' ', {'title':'Green',
 		\ 'items' : [{'filename':'F1', 'lnum':10}]})
+    call assert_equal(0, s)
     let qfcur = g:Xgetlist({'nr':0})
     call assert_true(qfcur.nr == qfprev.nr + 1)
     let l = g:Xgetlist({'items':1})