Update unreachable.rst: mention match exhaustiveness codes from checkers type … (#2062)

Author: wyattscarpenter

docs/guides/unreachable.rst

@@ -86,6 +86,14 @@ and is also present in ``typing_extensions`` starting at version 4.1.
 However, it is also possible to define a similar function in your own
 code, for example if you want to customize the runtime error message.
 
+This use of match statements for comprehensive matching is common
+enough that some typecheckers have direct support for checking all match
+statements for exhaustiveness, regardless of whether or not they have an
+``assert_never``-style function in their default arm. For instance, in pyright
+this can be enabled by enabling the diagnostic code ``reportMatchNotExhaustive``
+(on by default in strict mode), and in mypy it can be enabled by enabling
+the error code ``exhaustive-match``.
+
 You can also use ``assert_never()`` with a sequence of ``if`` statements:
 
 .. code:: python