bpo-23722: Fix docs for future __classcell__ changes. (GH-6999)

bpo-23722: Fix docs for future __classcell__ changes. (GH-6999) · python/cpython@10a122c

Original file line number	Diff line number	Diff line change
`@@ -1887,7 +1887,7 @@ current call is identified based on the first argument passed to the method.`
`1887`	`1887`	be propagated up to the ``type.__new__`` call in order for the class to be
`1888`	`1888`	`initialised correctly.`
`1889`	`1889`	Failing to do so will result in a :exc:`DeprecationWarning` in Python 3.6,
`1890`		- and a :exc:`RuntimeWarning` in the future.
	`1890`	+ and a :exc:`RuntimeError` in Python 3.8.
`1891`	`1891`
`1892`	`1892`	When using the default metaclass :class:`type`, or any metaclass that ultimately
`1893`	`1893`	calls ``type.__new__``, the following additional customisation steps are