[encoding] Clarify scope of document in preface (#10)

The Preface claims: 
> "… this specification … defines … the utf-8 encoding."

Isn't that formalised in ISO/IEC 10646:2014 and Unicode? I'm not suggesting that the contents of this document aren't useful, just that they don't define utf-8. 

I suggest resolving this issue by:

1. Removing "(and defines)" from the preface.
2. Adding a reference to ISO/IEC 10646 to the References section and referring to it in the definition of utf-8.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/encoding/issues/10