patch 9.1.0080: unexpected error for modifying final list using +=

Problem:  unexpected error for modifying final list using += operator
          (Ernie Rael)
Solution: Allow List value modification of a final variable using +=
          operator
          (Yegappan Lakshmanan)

fixes: #13745
fixes: #13959
closes: #13962

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 574b9c9..75ea2c7 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -1,4 +1,4 @@
-*eval.txt*	For Vim version 9.1.  Last change: 2024 Feb 05
+*eval.txt*	For Vim version 9.1.  Last change: 2024 Feb 06
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -425,6 +425,18 @@
 modified.  The value must at least have the number of items in the range: >
 	:let list[3:5] = [3, 4, 5]
 
+To add items to a List in-place, you can use the |+=| operator: >
+	:let listA = [1, 2]
+	:let listA += [3, 4]
+<
+When two variables refer to the same List, changing one List in-place will
+cause the referenced List to be changed in-place: >
+	:let listA = [1, 2]
+	:let listB = listA
+	:let listB += [3, 4]
+	:echo listA
+	[1, 2, 3, 4]
+<
 Adding and removing items from a list is done with functions.  Here are a few
 examples: >
 	:call insert(list, 'a')		" prepend item 'a'