Document Timeout behaviour

Adds better documentation of `timeout` behavior

- "How long to wait for the server to send data before giving up, as a float, or a (connect timeout, read timeout) tuple." was as short as I could get to accurately describe the behavior.

- Sphinx puts the description of timeouts on the line below the parameter name, but this is not true for other parameters. I'm curious why this is but I can't see any differences.

- We don't use the `stream` parameter anywhere in the `adapters.send` method, so I removed the "Behavior" section that described the behavior of requests incorrectly. May also want to remove the parameter description from the documentation, or state that it's not actually used anywhere.