Added comment to explain that explain should not return a list but a single message (divergence testers should not test multiple divergences at the same time if divergences can be resolved using different solvers).

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@30388 20353a03-c40f-0410-a6d1-a30d3c3de9de