docs: Clarify how to pass a custom set of CAs

This new wording makes it hopefully easier to find how to override the
system-provided list of trusted CAs. I failed to find this through some googling
and had to resort to asking on IRC previously.

docs/user/advanced.rst

@@ -166,7 +166,7 @@ I don't have SSL setup on this domain, so it fails. Excellent. GitHub does thoug
     >>> requests.get('https://github.com', verify=True)
     <Response [200]>
 
-You can also pass ``verify`` the path to a CA_BUNDLE file for private certs. You can also set the ``REQUESTS_CA_BUNDLE`` environment variable.
+You can pass ``verify`` the path to a CA_BUNDLE file with certificates of trusted CAs. This list of trusted CAs can also be specified through the ``REQUESTS_CA_BUNDLE`` environment variable.
 
 Requests can also ignore verifying the SSL certificate if you set ``verify`` to False.