Proxy docs changes:

- Move ftp-related questions to mod_proxy_ftp.

- Document the proxy-sendcl and proxy-sendchunked env variables.

- Improve the docs on the other proxy env variables.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@326130 13f79535-47bb-0310-9956-ffa450edef68