doc: fix incorrect documentation for ImageWrapper.getPixelsType (rebased onto dev_5_0)



This is the same as gh-2548 but rebased onto dev_5_0.

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The documentation for getPixelsType was incorrect (copy and paste from getPixelSizeX).

While doing it I added a bit more detail to what getPixelsType can be used for (otherwise one has to follow into the Java documentation which is not so obvious for someone without java knowledge).