Overriding the type map entry for embedded document does not work

If RegisterTypeMapEntry is used to decode into bson.M instead of bson.D for empty interface, the values still decode as bson.D.

Also, we should fix empty interface decoding to first check for ancestor information and then for type map entries. This will allow us to remove the warning from the RegisterTypeMapEntry documentation about registering a mapping for bsontype.EmbeddedDocument.