CTR: documentation and comments
Update CGI-howto.

Recommend to copy the servlet into one's own web application instead of uncommenting it globally.
The default value for cgiPathPrefix is null. The "WEB-INF/cgi" value is the recommended one and used in conf/web.xml example.

Merged r1640675 from tomcat/tc7.0.x/trunk.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc6.0.x/trunk@1640677 13f79535-47bb-0310-9956-ffa450edef68