patch 8.2.1255: cannot use a lambda with quickfix functions

Problem:    Cannot use a lambda with quickfix functions.
Solution:   Add support for lambda. (Yegappan Lakshmanan, closes #6499)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 71ee94b..6a9e2ed 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -5913,7 +5913,8 @@
 	customize the information displayed in the quickfix or location window
 	for each entry in the corresponding quickfix or location list.  See
 	|quickfix-window-function| for an explanation of how to write the
-	function and an example.
+	function and an example. The value can be the name of a function or a
+	lambda.
 
 	This option cannot be set from a |modeline| or in the |sandbox|, for
 	security reasons.