patch 9.0.1821: Vim9 constructors are always static

Problem:  Vim9 constructors are always static
Solution: make the "static" keyword an error

closes: #12945

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Gianmaria Bajo <mg1979.git@gmail.com>
diff --git a/runtime/doc/vim9class.txt b/runtime/doc/vim9class.txt
index 926638b..8a9e37e 100644
--- a/runtime/doc/vim9class.txt
+++ b/runtime/doc/vim9class.txt
@@ -315,6 +315,10 @@
 	    OtherThing._Foo()
 	enddef
     endclass
+<
+							*E1370*
+Note that constructors cannot be declared as "static", because they always
+are.
 
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