Please document UnixConnection work

[paultag]

My code is similar to:

where self.url is /run/foo.sock, and url is set to /run/foo.sock/events

         response = yield from aiohttp.request(
             method, url,
             connector=aiohttp.UnixSocketConnector(self.url), 
             headers=headers, data=data
         )

Which raises:

  File "/home/tag/dev/local/aiohttp/aiohttp/client.py", line 97, in request
    loop=loop, expect100=expect100)
  File "/home/tag/dev/local/aiohttp/aiohttp/client.py", line 177, in __init__
    self.update_host(url)
  File "/home/tag/dev/local/aiohttp/aiohttp/client.py", line 203, in update_host
    raise ValueError('Host could not be detected.')
ValueError: Host could not be detected.

From what I can tell from the test code, this should work, but better docs would be great, so that other people don't hit similar issues :) 👍

Thanks!

[fafhrd91]

added basic docs

[lock]

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