add description how to change channels to layer docs

[k-dominik]

We do have number 6 in our faq:

6. How can I view individual channels of my input data?
   2- and 3-channel data is loaded as RGB by default. To examine the channels one by one, you again have to go to the Dataset Properties dialog and change the Channel Display setting. After you do that, a tiny channel spinbox should appear at the Raw data layer in the lower left corner of the ilastik window.

However, the layer documentation is devoid of such mention of this layer control.