DOC: Fix documentation in VoronoiDiagram2DGenerator

To be parsed, all Doxygen comments must be above method declarations,
not definitions.

Change-Id: I3037e24352f0d0d49b78b6cb0fb73f8cb26e086a
Merge topic 'Doc-VoronoiDiagram'

68ba6169 DOC: Fix documentation in VoronoiDiagram2DGenerator