Document the Common Log Format

Document the new % directives (p, P, T, {}n)
Explain how to use multiple log files w/vhosts
Expand CustomLog definition
Add compat. note about CookieLog


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@77067 13f79535-47bb-0310-9956-ffa450edef68