Update js_image_arithmetics.markdown

doc/js_tutorials/js_core/js_image_arithmetics/js_image_arithmetics.markdown

@@ -27,7 +27,7 @@ src1.delete(); src2.delete(); dst.delete(); mask.delete();
 Image Subtraction
 --------------
 
-You can subtract two images by OpenCV function, cv.subtract(). res = img1 - img2. Both images should be of same depth and type.
+You can subtract two images by OpenCV function, cv.subtract(). res = img1 - img2. Both images should be of same depth and type. Note that when used with RGBA images, the alpha channel is also subtracted.
 
 For example, consider below sample:
 @code{.js}
@@ -59,4 +59,4 @@ Try it
 <iframe src="../../js_image_arithmetics_bitwise.html" width="100%"
         onload="this.style.height=this.contentDocument.body.scrollHeight +'px';">
 </iframe>
-\endhtmlonly
+\endhtmlonly