bpo-35581: Document @typing.type_check_only (GH-11312)

Doc/library/typing.rst

@@ -951,6 +951,24 @@ The module defines the following classes, functions and decorators:
    This wraps the decorator with something that wraps the decorated
    function in :func:`no_type_check`.
 
+.. decorator:: type_check_only
+
+   Decorator to mark a class or function to be unavailable at runtime.
+
+   This decorator is itself not available at runtime. It is mainly
+   intended to mark classes that are defined in type stub files if
+   an implementation returns an instance of a private class::
+
+      @type_check_only
+      class Response:  # private or not available at runtime
+          code: int
+          def get_header(self, name: str) -> str: ...
+
+      def fetch_response() -> Response: ...
+
+    Note that returning instances of private classes is not recommended.
+    It is usually preferable to make such classes public.
+
 .. data:: Any
 
    Special type indicating an unconstrained type.

Misc/NEWS.d/next/Documentation/2018-12-25-12-56-57.bpo-35581.aA7r6T.rst

+@typing.type_check_only now allows type stubs to mark functions and classes not available during runtime.
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