PEP 636: clarify __match_args__ naming (#2136)

* clarify __match_args__ naming

The current example allows the reader to wonder whether the __match_args__ names must match the __init__ args or the class members.  This change explicitly shows that class member names are searched for the __match_args__ items (admittedly at the cost of some clutter).

* Update pep-0636.rst

Thx Guido, that reduces the clutter.

Co-authored-by: Guido van Rossum <gvanrossum@gmail.com>

* oops, thx again!

Co-authored-by: Guido van Rossum <gvanrossum@gmail.com>

pep-0636.rst

@@ -359,7 +359,9 @@ this alternative definition::
 
     class Click:
         __match_args__ = ("position", "button")
-        def __init__(self, position, button):
+        def __init__(self, pos, btn):
+            self.position = pos
+            self.button = btn
             ...
 
 The ``__match_args__`` special attribute defines an explicit order for your attributes