patch 8.0.1844: superfluous quickfix code, missing examples

Problem:    Superfluous quickfix code, missing examples.
Solution:   Remove unneeded code.  Add a few examples.  Add a bit more
            testing. (Yegappan Lakshmanan, closes #2916)
diff --git a/runtime/doc/quickfix.txt b/runtime/doc/quickfix.txt
index 83379b8..5f2a4dd 100644
--- a/runtime/doc/quickfix.txt
+++ b/runtime/doc/quickfix.txt
@@ -608,6 +608,12 @@
     " get the identifier of the current quickfix list
     :let qfid = getqflist({'id' : 0}).id
 
+    " get the identifier of the fourth quickfix list in the stack
+    :let qfid = getqflist({'nr' : 4, 'id' : 0}).id
+
+    " check whether a quickfix list with a specific identifier exists
+    :if getqflist({'id' : qfid}).id == qfid
+
     " get the index of the current quickfix list in the stack
     :let qfnum = getqflist({'nr' : 0}).nr
 
@@ -653,6 +659,11 @@
 attributes of a quickfix and location list respectively. Some examples for
 using these functions are below:
 >
+    " create an empty quickfix list with a title and a context
+    :let t = 'Search results'
+    :let c = {'cmd' : 'grep'}
+    :call setqflist([], ' ', {'title' : t, 'context' : c})
+
     " set the title of the current quickfix list
     :call setqflist([], 'a', {'title' : 'Mytitle'})
 
@@ -671,6 +682,9 @@
 		    \ {'filename' : 'b.txt', 'lnum' : 20, 'text' : "Orange"}]
     :call setqflist([], 'a', {'id' : qfid, 'items' : newItems})
 
+    " empty a quickfix list specified by an identifier
+    :call setqflist([], 'r', {'id' : qfid, 'items' : []})
+
     " free all the quickfix lists in the stack
     :call setqflist([], 'f')