conditional require

During recent major version update, some files were moved from <package>-doc to <package>, and as a result updates of <package> fail due to a file conflict. Manual update of <package>-doc resolves this, of course. A simple solution would be to declare that <package> Required: package-doc >= 6.0.0 but that would force the install of the docs package if it wasn't already there. Is there a way to declare a dependency only if the other package is present/installed? Would Obsoletes: package-doc < 6.0.0 be the right thing to do?