Need docs to claim that all timeouts will not change during sessions

[defia]

some options in request options, such as all timeout settings, is actually bind to http.Client.
in doRegularRequest, we create client each time if timeout is not zero.
but in doSessionRequest, the client is created in NewSession().
so if user pass a RequestOptions with timeout to session.Get(), it will not change client's timeout.

I think current timeout implementation is good,but it's better to add some docs about this behavior.

[ghost]

1. connet timeout
2. read response data timeout.
3. timeout must be a range int