patch 9.0.1760: vim9 class problem with new() constructor

Problem:  vim9 class problem with new() constructor
Solution: Don't allow a return type for the new() class constructor.

closes: #12863
closes: #12040

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>
diff --git a/runtime/doc/vim9class.txt b/runtime/doc/vim9class.txt
index 6acbca5..a68b97c 100644
--- a/runtime/doc/vim9class.txt
+++ b/runtime/doc/vim9class.txt
@@ -232,6 +232,9 @@
 step the members of the parent class are done first.  There is no need to call
 "super()" or "new()" on the parent.
 
+When defining the new() method the return type should not be specified.  It
+always returns an object of the class.
+
 ==============================================================================
 
 3.  class members and functions			*Vim9-class-member*