collection, custom_id, object_id and raw_type types added to basic-mapping list

The basic mapping type in basic-mapping.rst did not seem to be up
to date with the MongoDB Types defined in Doctrine/ODM/MongoDB
(https://github.com/mapado/mongodb-odm/tree/master/lib/Doctrine/ODM/MongoDB/Types).

The following types were added:
 * ``collection``
 * ``custom_id``
 * ``object_id``
 * ``raw_type``

Also, a quick note was added on which MongoDB type the ``collection``
type is mapped to.

Signed-off-by: Balthazar Rouberol <balthazar.rouberol@mapado.com>