webhooks: Update comment about typing the webhook decorator.
webhooks: Update comment about typing the webhook decorator.

The previous link was to "extended callable" types, which are
deprecated in favor of callback protocols.  Unfortunately, defining a
protocol class can't express the typing -- we need some sort of
variadic generics[1].  Specifically, we wish to support hitting the
endpoint with additional parameters; thus, this protocol is
insufficient:

```
class WebhookHandler(Protocol):
    def __call__(request: HttpRequest, api_key: str) -> HttpResponse: ...
```
...since it prohibits additional parameters.  And allowing extra
arguments:
```
class WebhookHandler(Protocol):
    def __call__(request: HttpRequest, api_key: str,
                 *args: object, **kwargs: object) -> HttpResponse: ...
```
...is similarly problematic, since the view handlers do not support
_arbitrary_ keyword arguments.

[1] https://github.com/python/typing/issues/193

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