Add a little documentation on how to select DBMs during build

Add a little documentation on how to select DBMs during build
and update the RerwiteMap and AuthDBMType directives to point
to it.

The build docs could still use a great deal of work.  There
are tons of things that aren't documented at all.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@96526 13f79535-47bb-0310-9956-ffa450edef68