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.
Need to add a `~` if you add a character.