Redo 'controlling access to proxy' section, noting that Limit block is

required, that Files block is required for versions < 1.2b10, and using
all allowable methods in the Limit block.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@78128 13f79535-47bb-0310-9956-ffa450edef68