Update explainer.md with changes related to use of -1 (#641)

PointerEventDeviceId/explainer.md

@@ -29,9 +29,10 @@ Microsoft's Surface Hub device supports detection of multiple pen devices intera
 ## Proposed Solution
 The proposed solution is to add a new attribute `deviceId` to `PointerEvent` that has the following characteristics:
 
-1. The attribute will be populated if both the digitizer and the pen support getting a unique hardware ID for the pen.
-1. The attribute will be `null` if the digitizer and pen support getting a unique hardware ID, but the ID is not available during the current event due to limitations (see [Limitations of Current Hardware](#limitations-of-current-hardware)).
-1. The attribute will be `undefined` if either the digitizer or the pen do not support getting a unique hardware ID.
+1. The attribute will be populated if both the digitizer and the pen support getting a unique hardware ID for the pen, with a value of 2 or more.
+1. The attribute will be `-1` if the digitizer and pen support getting a unique hardware ID, but the ID is not available during the current event due to limitations (see [Limitations of Current Hardware](#limitations-of-current-hardware)).
+1. The attribute will be `-1` if either the digitizer or the pen do not support getting a unique hardware ID.
+1. Like pointerId, the attribute will be 1 if the pointer event is generated by mouse.
 
 ### Web IDL
 The proposed WebIDL for this feature is as follows:
@@ -63,7 +64,7 @@ This ID allows the developer to assign a different inking color to each unique p
             // Listen for a `pointerdown` event and map the deviceId to a color if it exists
             // and has not been mapped yet.
             canvas.addEventListener('pointerdown', function(e) {
-                if (e.deviceId && !pen_to_color_map.has(e.deviceId)) {
+                if (e.deviceId && (e.deviceId > 1) && !pen_to_color_map.has(e.deviceId)) {
                     pen_to_color_map.set(e.deviceId, COLORS[color_assignment_index]);
 
                     // Bump the color assignment index and loop back over if needed
@@ -90,9 +91,9 @@ The ability to gather a unique hardware ID from the pen is a fairly new capabili
 
 | Scenario | Value of `deviceId` |
 | :- | :- |
-| Digitizer and Pen support Hardware ID on OS | `unsigned long` |
-| Digitizer and Pen support Hardware ID, but not for initial contact | `null` |
-| No support for Hardware ID | `undefined` |
+| Digitizer and Pen support Hardware ID on OS | `signed long` |
+| Digitizer and Pen support Hardware ID, but not for initial contact | `-1` |
+| No support for Hardware ID | `-1` |
 
 ## Privacy and Security Considerations
 ### Privacy