WP_HTTP: Clean up some comments style and remove outdated information.
The change to the proxy block here is for consistency, WP_HTTP_Proxy::block_request() is never called before parse_url() has successfully operated on the URL, and in the event we cannot read the URL, it shouldn't be requested.

Built from https://develop.svn.wordpress.org/trunk@25221


git-svn-id: http://core.svn.wordpress.org/trunk@25192 1a063a9b-81f0-0310-95a4-ce76da25c4cd
WP_HTTP: Add a filter for the available HTTP Transports to allow plugins to alter the order which HTTP Transports are tried, or to add their own.

Built from https://develop.svn.wordpress.org/trunk@25222


git-svn-id: http://core.svn.wordpress.org/trunk@25193 1a063a9b-81f0-0310-95a4-ce76da25c4cd