document is considered broken by recent OOo versions.

addImage accept new argument content_type which must be required.
But backward compatibility is kept so FutureWarning is raised instead
to inform user to update the argument list.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@40555 20353a03-c40f-0410-a6d1-a30d3c3de9de