Add https.proxy* documentation to HttpClientBuilder

To amend HTTPCLIENT-1128 I guess...

I find the list of http.proxy* system properties in the type Javadoc for HttpClientBuilder a bit misleading.

The JVM distinguishes between HTTP and HTTPS proxy settings (decide for yourself how much sense that makes) but the Javadoc comment only talks about http.proxy* system properties and not https.proxy* system properties. One might come to the conclusion that by using #useSystemProperties() on the builder you'd only get proxy support for HTTP connections but not for HTTPS.

Adding a short note about that would be appropriate IMO.