Add a hint about how CONNECT-method requests are filtered out by <Proxy>,

Add a hint about how CONNECT-method requests are filtered out by <Proxy>,
and how to enable them. I could not find this information easily.

In particular, the straightforward <Proxy https://www.example.com>... does
not allow CONNECT to the corresponding SSL server. Probably it would work
if a web client sent a "GET https://www.example.com/ HTTP/1.1" to the
proxy, but none would do that.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1648846 13f79535-47bb-0310-9956-ffa450edef68