[selection-api] Clarify the additions of this spec to GlobalEventHandlers (#54)

It seems to me that this spec needs to include the following IDL changes for the event handlers that it adds, something like:

```
partial interface GlobalEventHandlers {
  attribute EventHandler onselectstart;
  attribute EventHandler onselectionchange;
};
```

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/selection-api/issues/54