Explicitly control layout of tables generated for parameters of mojo description

The handling for the <table> element in commit 578469 to doxia-module-xdoc will cause a change for the current output of the PluginXdocGenerator (i.e. table cells get centered instead of left aligned and a border will be rendered). Updating the XDoc generator now should allow a smooth transition to doxia 1.0-beta-1.