[3.7] bpo-36985: Document typing.ForwardRef (GH-14216) (GH-14220)

https://bugs.python.org/issue36985

Author: ilevkivskyi

Doc/library/typing.rst

@@ -876,6 +876,13 @@ The module defines the following classes, functions and decorators:
    .. versionchanged:: 3.6.1
       Added support for default values, methods, and docstrings.
 
+.. class:: ForwardRef
+
+   A class used for internal typing representation of string forward references.
+   For example, ``List["SomeClass"]`` is implicitly transformed into
+   ``List[ForwardRef("SomeClass")]``.  This class should not be instantiated by
+   a user, but may be used by introspection tools.
+
 .. function:: NewType(typ)
 
    A helper function to indicate a distinct types to a typechecker,