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

[3.7] bpo-36985: Document typing.ForwardRef (GH-14216) (GH-14220) · python/cpython@d058735

Original file line number	Diff line number	Diff line change
`@@ -876,6 +876,13 @@ The module defines the following classes, functions and decorators:`
`876`	`876`	`.. versionchanged:: 3.6.1`
`877`	`877`	`Added support for default values, methods, and docstrings.`
`878`	`878`
	`879`	`+.. class:: ForwardRef`
	`880`	`+`
	`881`	`+ A class used for internal typing representation of string forward references.`
	`882`	+ For example, ``List["SomeClass"]`` is implicitly transformed into
	`883`	+ ``List[ForwardRef("SomeClass")]``. This class should not be instantiated by
	`884`	`+ a user, but may be used by introspection tools.`
	`885`	`+`
`879`	`886`	`.. function:: NewType(typ)`
`880`	`887`
`881`	`888`	`A helper function to indicate a distinct types to a typechecker,`