DOC: Clarified NormalizedCorrelationImageToImageMetric value

Thanks to Jonas Beuke for reporting the issue.

Also fixed a few style issues in the test for this class.

Change-Id: I2cb15fc801dee4a6276eda7b43d7b83376e20d13
Merge topic 'Doc-NormCorrelMetric'

7055107 DOC: Clarified NormalizedCorrelationImageToImageMetric value