patch 8.2.0935: flattening a list with existing code is slow

Problem:    Flattening a list with existing code is slow.
Solution:   Add flatten(). (Mopp, closes #3676)
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 1d896f9..e39f1fe 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -650,6 +650,7 @@
 	min()			minimum value in a List
 	count()			count number of times a value appears in a List
 	repeat()		repeat a List multiple times
+	flatten()		flatten a List
 
 Dictionary manipulation:				*dict-functions*
 	get()			get an entry without an error for a wrong key