Improve NodeTypeManager.registerNodeType methods

Make the ModeShape-specific NodeTypeManager.registerNodeTypes(...) methods more like the standard javax.jcr.nodetype.NodeTypeManager.registerNodeTypes(...) method. In particular, update the JavaDoc and change the return type from void to NodeTypeIterator.

These changes are backward compatible with the changes in the API added to ModeShape 2.7. They are also what I've proposed to JSR-333 with the JSR-333-46 feature request.