Add more info about the role of Timeout for mod_cgi

PR 60404 asks if the Timeout directive is related to
the amount of time to wait for the entire CGI script's
output or only the first byte received.
Checking in mod_cgi.c -> cgi_bucket_read shows a
apr_pollset_poll that should support the latter.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1780907 13f79535-47bb-0310-9956-ffa450edef68