[w3c/touch-events] Clarify "deprecated in this specification" (#80)

Our wording [here](https://w3c.github.io/touch-events/#extensions-to-the-document-interface) is vague.  We should probably say explicitly that they're included for compatibility but usage is discouraged as they are expected to be removed in the future.  In particular, I think we're pretty close to being able to remove them from Chrome - once that's been successful we'll discuss removal from the spec.

/cc @foolip

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