Update command-line/docs to mention --timeout-keep-alive is 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:

uvicorn/uvicorn/protocols/http/h11_impl.py

Line 318 in c1144fd

self.timeout_keep_alive_task = self.loop.call_later(self.timeout_keep_alive, self.timeout_keep_alive_handler)

and here:

uvicorn/uvicorn/protocols/http/httptools_impl.py

Lines 332 to 334 in c1144fd

	self.timeout_keep_alive_task = self.loop.call_later(
	self.timeout_keep_alive, self.timeout_keep_alive_handler
	)

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.