DOC: Update GDCMImageIO rescale slope intercept doc.

There was a not that GDCMImageIO was not handling rescale slope/intercept
properly, and that this was fixed on 11/12/2007.  However, it broke again,
and it was fixed before the ITK 4.6.0 release.

Remove the note.  Also fix up some grammar.

Change-Id: I4d2600b02bdb8cae8f6ba9dae96d02f4338d4ca1
Merge topic 'GDCMImageIORescaleSlopeDoc2'

dddc9ed6 DOC: Update GDCMImageIO rescale slope intercept doc.