DOC: No ReferenceImage members in TransformToDisplacementFieldSource.

Note that this class is deprecated in favor of
TransformToDisplacementFieldFilter where these members do exist.

Issue ITK-3247.

Change-Id: Ie30af0e48e0cf318ff05f29fbfefa27d34cc65fa
Merge topic 'DisplacementFieldSourceDoc'

425f0316 DOC: No ReferenceImage members in TransformToDisplacementFieldSource.