Allow additional 'maven.docs.src' directories to be specified

Assume a directory structure like the following:

common/
xdoc/
foo/
project.xml
project.properties
project1/
...
xdoc/
project.xml
project.properties
project2/
...
xdoc/
project.xml
project.properties

where "projectN" extends "common". The "common/xdoc/foo" documentation applies to all "projectN"; I don't want to duplicate it, so it lives in "common". However, in order to get it included in the site for "projectN", I have to have a preGoal on xdoc:copy-user-resources to copy it to "projectN/xdoc/foo". This results in a needless copy and it causes problems with SVN marking these copied files as needing to be added to the repository.

I'd like to see one of two enhancements (or both):
1. Add a property ('maven.docs.src.additional', or whatever), which would specify a list of additional directories that would be included in the xdoc transformation; this would prevent the copy and the SVN confusion.
2. By default, if a project extends another project, look for, and include, the xdoc of the extended project in the extending projects xdoc transformation.