ACTION-27: Propose text re the "spec's positioning" (see LC comment from Alex Russell)

Hey folks,

In our March 12 call, we decided we should add some non-normative text to clarify the spec's positioning [1]. Here's my proposed text:

While Pointer Events are sourced from a variety of input devices, they are not defined as being generated from some other set of device-specific events. While possible and encouraged for compatibility, this spec does not require other device-specific events be supported (e.g. mouse events, touch events, etc.).  A user agent could support pointer events without supporting any other device events. For compatibility with content written to mouse-specific events, this specification does provide an option section describing how to generate compatibility mouse events based on pointer input from devices other than a mouse.

Feel free to give suggested tweaks. This text would be for the informative introduction.

-Jacob