Refresh parts of the proxy docs, with special attention to clarifying the

Refresh parts of the proxy docs, with special attention to clarifying the
security issues in forward and reverse proxy situations.

- Remove the historical narrative in the top section and simply state
the facts.

- Remove the table of contents, since we already generate one for the sidebar.

- Rewrite the forward vs reverse section to be clear on the purpose of
the two types of proxy.

- Add some basic examples of each type of proxy to replace the example removed
from the default config file.

- Add a note to the access control section on forward versus reverse proxys,
and also add a reference to mod_access that got removed at some point.

- Add a warning to the ProxyPass docs that ProxyRequests should be off.

- Shorten some section titles.

I did not add the
ProxyBlock :25
because I don't have a proxy server to test it on at the moment.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@100759 13f79535-47bb-0310-9956-ffa450edef68