Add comment reminding of the need for a gutter

Needed to properly implement abort on move outside keyboard (1f1914d)

Author: yawnoc

app/src/main/java/io/github/yawnoc/strokeinput/InputContainer.java

@@ -238,6 +238,9 @@ private static int getContrastingColour(final int colour) {
 
   /*
     Handle logic for multiple pointers (e.g. two-thumb typing).
+    The correct handling of a pointer moving outside the keyboard
+    is ensured by including a 1-pixel gutter at the top of the keyboard
+    (so that the pointer must move through a key-free row of pixels).
     // TODO: fix unexpected behaviour when two pointers merge
   */
   @SuppressLint("ClickableViewAccessibility")