Improve documentation about imread and cvtColor function behaviour (Bug #2282)

"imread" documentation does not clearly states how RGB pixel data is stored in a Mat after loading an image. Since it is not stored in a intuitive manner (in red, green, blue order), I believe that this should be present in its page (which, I think, is this one: http://docs.opencv.org/modules/highgui/doc/reading_and_writing_images_and_video.html#imread).

Also, documentation about converting RGB to HLS with function cvtColor (http://docs.opencv.org/modules/imgproc/doc/miscellaneous_transformations.html#cvtcolor) should also be fixed. It seems that the section that explains how H, L, and S values are converted to 8, 16 or 32 bits values has been copied from the RGB to HSV equivalent section. Also, I think you should add in this function a statement about the order those values are stored in a Mat object.