I don't know if it's documented anywhere. I found out the hard way ;-) by adding the line without changing the start script. I had to ssh into the server to kill the Zope process before it could finish booting. Once I removed the -D argument it worked fine.
I have put a note in the "Maintaining Zope" chapter at http://www.zope.org/Documentation/Books/ZopeBook/2_6Edition/Maintain ingZope.stx to the effect that we need to have better docs about this.
I'm tempted--despite how relatively clueless I am--to write yet another HOWTO describing my experiences getting Zope working with Apache. I suppose it'd be more effective to try to contribute to some canonical reference, though. Er, I mean, find, read, and grok it first, of course. Is there such a thing?
You may want to read http://www.zope.org/Documentation/Books/ZopeBook/2_6Edition/Maintain ingZope.stx/document_src in your own copy add anything you think is worthwhile, sending the result to me in email for inclusion in the Book...