patch 9.0.1053: default constructor arguments are not optional

Problem:    Default constructor arguments are not optional.
Solution:   Use "= v:none" to make constructor arguments optional.
diff --git a/runtime/doc/vim9class.txt b/runtime/doc/vim9class.txt
index 7785fbe..4dc84f3 100644
--- a/runtime/doc/vim9class.txt
+++ b/runtime/doc/vim9class.txt
@@ -431,15 +431,15 @@
 
 Then The default constructor will be: >
 
-	def new(this.name = void, this.age = void, this.gender = void)
+	def new(this.name = v:none, this.age = v:none, this.gender = v:none)
 	enddef
 
 All object members will be used, also private access ones.
 
-The "= void" default values make the arguments optional.  Thus you can also
-call `new()` without any arguments.  Since "void" isn't an actual value, no
-assignment will happen and the default value for the object members will be
-used.  This is a more useful example, with default values: >
+The "= v:none" default values make the arguments optional.  Thus you can also
+call `new()` without any arguments.  No assignment will happen and the default
+value for the object members will be used.  This is a more useful example,
+with default values: >
 
 	class TextPosition
 	   this.lnum: number = 1
@@ -450,8 +450,12 @@
 yourself.  For example, if for the AutoNew class above you insist on getting
 the name, you can define the constructor like this: >
 
-	def new(this.name, this.age = void, this.gender = void)
+	def new(this.name, this.age = v:none, this.gender = v:none)
 	enddef
+<							*E1328*
+Note that you cannot use another default value than "v:none" here.  If you
+want to initialize the object members, do it where they are declared.  This
+way you only need to look in one place for the default values.
 
 
 Multiple constructors ~