Merge r1298433, r1305166 from trunk:

Use new ap_runtime_dir_relative() API

Document use of the DefaultRuntimeDir directive.

Submitted by: jim, minfrin
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1308897 13f79535-47bb-0310-9956-ffa450edef68