general mod_proxy docs cleanup:

- add documentation for ProxyIOBufferSize (review desired)
- fix context lists of ProxyPass and ProxyPassReverse
- add more notes about the accompanying modules (_http, _ftp, _connect)
- markup & reformatting (sorry for the big diff)
-> update transformation


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@99725 13f79535-47bb-0310-9956-ffa450edef68