Improve documentation around customizing handling of handshake

[cjerdonek]

I was trying to figure out how to return a 404 on a bad path. Two things occur to me that would have made this a bit easier to figure out.

First, the end of the docstring of the server's handshake() method could be expanded to say, "Return the URI of the request, or None if the handshake was aborted."

Second, the docstring of the server's get_response_status() method could be expanded to say that the request path (e.g. to decide whether to return a 404) is available at self.path when that method is called. Currently, you have to notice that read_http_request() is called at the beginning of handshake() and then see that read_http_request() sets self.path. But even then, it's not clear if you'd be relying on implementation details.

[aaugustin]

Go for it and make a PR!

[cjerdonek]

Okay, great. Though if you're open to my comment here (which I wrote later), this issue would probably be subsumed.

[aaugustin]

Since #248 was merged, it's time to reopen the discussion here!

[aaugustin]

The section about "Port sharing" in deployment.rst should be updated, since it is now possible to implement basic HTTP responses.

[aaugustin]

Since I restructured the docs, the best place to put this information would be in its own page, linked from the "howto" section in index.rst.

[aaugustin]

By now, I believe this issue has become a duplicate of #354 for which I submitted PR #387.

The original issue is "I was trying to figure out how to return a 404 on a bad path."

The modern API for doing that is process_request, like this:

    async def process_request(self, path, request_headers):
        if path != '/':
            return http.HTTPStatus.NOT_FOUND, [], b'sorry not found\n'

This is close enough to the example in #387.