patch 9.0.2182: Vim9: need a way to reserve future extension

Problem:  Vim9: need a way to reserve future extension
Solution: reserve double underscore prefix for future use
          (Yegappan Lakshmanan)

related: #13238
closes: #13742

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/vim9class.txt b/runtime/doc/vim9class.txt
index 4dc67bd..929597c 100644
--- a/runtime/doc/vim9class.txt
+++ b/runtime/doc/vim9class.txt
@@ -364,6 +364,9 @@
 corresponding class member will be used.  The type of the class member in a
 child class can be different from that in the super class.
 
+The double underscore (__) prefix for a class or object method name is
+reserved for future use.
+
 					*object-final-variable* *E1409*
 The |:final| keyword can be used to make a class or object variable a
 constant.  Examples: >