"{c}h" exists and logs the value returned by 'ap_get_useragent_host()'.

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 r1730129.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1818321 13f79535-47bb-0310-9956-ffa450edef68