Update CGI-howto.
Recommend to copy the servlet into one's own web application instead of uncommenting it globally.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1640655 13f79535-47bb-0310-9956-ffa450edef68
Correct documentation for CGI servlet:
The default value for cgiPathPrefix is null. The "WEB-INF/cgi" value is the recommended one and used in conf/web.xml example.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1640658 13f79535-47bb-0310-9956-ffa450edef68