DOC: Doxygen expansion of itkGetModifiableObjectMacro.

Also fix bug in Doxygen definition of itkGetConstObjectMacro. (not listed as
const).

Change-Id: Ic7ade94a71d825f815f2de29ea8b11dc79f4a5a2
Merge topic 'ModifiableObjectMacro3'

6f1c432 DOC: Doxygen expansion of itkGetModifiableObjectMacro.