Add note about --timeout-keep-alive being measured in seconds

[LincolnPuzey]

Summary

Update command-line/docs to mention --timeout-keep-alive is measured in seconds.
I believe this is correct since this value is ultimately passed to loop.call_later

Here:
https://github.com/encode/uvicorn/blob/c1144fd4f130388cffc05ee17b08747ce8c1be11/uvicorn/protocols/http/h11_impl.py#L318

and here:
https://github.com/encode/uvicorn/blob/c1144fd4f130388cffc05ee17b08747ce8c1be11/uvicorn/protocols/http/httptools_impl.py#L332-L334

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.