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/quickfix.txt b/runtime/doc/quickfix.txt
index 293014b..4877648 100644
--- a/runtime/doc/quickfix.txt
+++ b/runtime/doc/quickfix.txt
@@ -1964,7 +1964,10 @@
 window for each entry from start_idx to end_idx. The function can obtain
 information about the entries using the |getqflist()| function and specifying
 the quickfix list identifier "id". For a location list, getloclist() function
-can be used with the 'winid' argument.
+can be used with the 'winid' argument. If an empty list is returned, then the
+default format is used to display all the entries. If an item in the returned
+list is an empty string, then the default format is used to display the
+corresponding entry.
 
 If a quickfix or location list specific customization is needed, then the
 'quickfixtextfunc' attribute of the list can be set using the |setqflist()| or