Backport from trunk: Add a note that VirtualDocumentRoot will override...

Backport from trunk: Add a note that VirtualDocumentRoot will override DocumentRoot in the current scope (and thus also in all virtual hosts if defined in the global server scope)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1349158 13f79535-47bb-0310-9956-ffa450edef68