Merge r1599535, r1721685 from trunk:

add GlobalLog directive to allow a diagnostic log to be inherited
by all virtual hosts, even if they define their own logs.

Submitted By: Edward Lu <Chaosed0 gmail.com>
Committed by: covener



document GlobalLog

Submitted by: covener
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1722340 13f79535-47bb-0310-9956-ffa450edef68