"{c}L" exists and logs the "Log-Id of the Connection".

I'm not sure of the description to write there, so I just mention it in the doc and hope s.o. will find the words :)

This has been introduced in r992806 and is also part of 2.4.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1818323 13f79535-47bb-0310-9956-ffa450edef68