patch 8.2.0869: it is not possible to customize the quickfix window contents

Problem:    It is not possible to customize the quickfix window contents.
Solution:   Add 'quickfixtextfunc'. (Yegappan Lakshmanan, closes #5465)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index b753aae..fb0b2a7 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -5508,8 +5508,9 @@
 			id	get information for the quickfix list with
 				|quickfix-ID|; zero means the id for the
 				current list or the list specified by "nr"
-			idx	index of the current entry in the quickfix
-				list specified by 'id' or 'nr'.
+			idx	get information for the quickfix entry at this
+				index in the list specified by 'id' or 'nr'.
+				If set to zero, then uses the current entry.
 				See |quickfix-index|
 			items	quickfix list entries
 			lines	parse a list of lines using 'efm' and return
@@ -5545,7 +5546,7 @@
 				If not present, set to "".
 			id	quickfix list ID |quickfix-ID|. If not
 				present, set to 0.
-			idx	index of the current entry in the list. If not
+			idx	index of the quickfix entry in the list. If not
 				present, set to 0.
 			items	quickfix list entries. If not present, set to
 				an empty list.
@@ -8841,6 +8842,11 @@
 		    nr		list number in the quickfix stack; zero
 				means the current quickfix list and "$" means
 				the last quickfix list.
+		    quickfixtextfunc
+				function to get the text to display in the
+				quickfix window.  Refer to
+				|quickfix-window-function| for an explanation
+				of how to write the function and an example.
 		    title	quickfix list title text. See |quickfix-title|
 		Unsupported keys in {what} are ignored.
 		If the "nr" item is not present, then the current quickfix list