Removed incorrect release of obtained colorspace. This fixes bug #3318 (updated commit for 2.4 branch)

Making a CGRelease* in Core Graphics is only allowed if you created or retained the object in question. This removes incorrect release of obtained colorspace.

And also cleaned up the documentation somewhat.

This fixes bug: http://code.opencv.org/issues/3318

New commit for 2.4 as requested by @apavienko (https://github.com/Itseez/opencv/pull/1671)