text: Title: `raise_for_status` now raises an exception for 1xx and 3xx responses
  Body: Hi!
  
  I've just noticed that `response.raise_for_status()` now raises an exception for 1xx and 3xx requests. I don't think it was the case before (namely with version 0.18) and the documentation doesn't seem to have been updated here: https://www.python-httpx.org/exceptions/
  
  > The HTTPStatusError class is raised by response.raise_for_status() on 4xx and 5xx responses.
  
  ### Steps to reproduce (even though I think this is an intentional behavior, the doc is just not up to date?)
  
  ```python
  import httpx
  url_301 = "https://stat.pagesjaunes.fr/redirect?target=ETvxvbcLWawl69YzLe_Ph4vo03E_Yj6_Jt1_EvjjbHFaCE3c9RGLEg==&v=2.0&p1=1&p2=1&p3=&p4=RESEAUSOC-SITE-LVS-GRATUIT&p5=LR&p6=1361647273529289141447273529289&p7=06486910&p8="
  httpx.get(url_301, follow_redirects=False).raise_for_status()