need for hook_rdf_mapping_alter

In developing documentation for the module, I realized that there is an edge case which may necessitate a hook_rdf_mapping_alter.

1. A module defines a content type and a mapping for that content type
   using hook_rdf_mapping
2. Another module alters that mapping
   using hook_entity_info_alter
3. The user wants to change the rdf mapping
   using the UI

In this case, when the user saves the new mapping, it is stored in the database. It is then overridden by the second module's mapping when the node is being displayed.

Instead, the second module's mapping should also be saved in the database so that the user can have full control over the final mapping.