patch 8.2.3848: cannot use reduce() for a string

Problem:    Cannot use reduce() for a string.
Solution:   Make reduce() work with a string. (Naruhiko Nishino, closes #9366)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 6765bdc..d54523f 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -8959,9 +8959,9 @@
 
 reduce({object}, {func} [, {initial}])			*reduce()* *E998*
 		{func} is called for every item in {object}, which can be a
-		|List| or a |Blob|.  {func} is called with two arguments: the
-		result so far and current item.  After processing all items
-		the result is returned.
+		|String|, |List| or a |Blob|.  {func} is called with two arguments:
+		the result so far and current item.  After processing all
+		items the result is returned.
 
 		{initial} is the initial result.  When omitted, the first item
 		in {object} is used and {func} is first called for the second
@@ -8972,6 +8972,7 @@
 			echo reduce([1, 3, 5], { acc, val -> acc + val })
 			echo reduce(['x', 'y'], { acc, val -> acc .. val }, 'a')
 			echo reduce(0z1122, { acc, val -> 2 * acc + val })
+			echo reduce('xyz', { acc, val -> acc .. ',' .. val })
 <
 		Can also be used as a |method|: >
 			echo mylist->reduce({ acc, val -> acc + val }, 0)