doc: lack of detail in Device datamodel

In https://github.com/telefonicaid/iotagent-node-lib/blob/master/doc/api.md#config-group-datamodel

• type should be detailed
• Description should be improved (e.g. "list" is ambiguous, it should be explained that it refers to a list of objects)

In https://github.com/telefonicaid/iotagent-node-lib/blob/master/doc/api.md#device-datamodel

• Description should be improved. It is not explained with fields can be used in the elements of such lists. Consider to use an extra section for that if needed

Side-note: why in config group we use eg. string as type but in device we use string as type? It should be the same style (I'd suggest just eg. string for simplicity)