added extra important warning

Small notice that could avoid many problems
More info http://code.opencv.org/issues/3802
Well, looks nice, but I'm not sure if the message is descriptive enough. http://code.opencv.org/issues/3802 mentions some more details. We can replace the text with something like:

_Training parameters are loaded from the parameter file in data folder! Please empty the data folder of you want to use your own parameters._

Or something like this. I think we can put more some more details to better instruct user.