patch 9.0.0092: plugins cannot change v:completed_item

Problem:    Plugins cannot change v:completed_item.
Solution:   Make v:completed_item writeable. (Shougo Matsushita,
            closes #10801)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 290f03f..3dfacf7 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2005,6 +2005,8 @@
 		|Dictionary| containing the |complete-items| for the most
 		recently completed word after |CompleteDone|.  The
 		|Dictionary| is empty if the completion failed.
+		Note: Plugins can modify the value to emulate the builtin
+		|CompleteDone| event behavior.
 
 					*v:count* *count-variable*
 v:count		The count given for the last Normal mode command.  Can be used