text: Title: add docs about Client Side Certificates for 2-way ssl
  Body: I was expecting it in https://www.python-httpx.org/advanced/#ssl-certificates section. 
  
  I went to read the source code expecting parallel api like [this](https://stackoverflow.com/questions/9093289/how-to-create-a-dual-authentication-https-client-in-python-without-lgpl-libs). And I found it supported. 
  
  I search the docs again but it is mentioned on https://www.python-httpx.org/compatibility/#ssl-configuration and https://www.python-httpx.org/api/#helper-functions
  
  I think it would be nice if the **Advanced/SSL Section** mention about this.