Rename RelationList criterion to FieldRelation

This PR relates to https://github.com/ezsystems/ezpublish-kernel/pull/547

As this criterion is capable of handling all field relations (all field types storing `eZ\Publish\API\Repository\Values\Content\Relation::FIELD` type relations - in standard installation these are Relation and RelationList field types at the moment), it is renamed to reflect this.

Later we might want to implement ContentRelation or general Relation criterion.

Additional: small improvement to FieldRelation handler class description.