[whatwg/fetch] Accept headers in spec cause confusion (#274)

During recent [Chrome patch discussions](https://codereview.chromium.org/1839473002/#msg56), concerns were raised that Chrome's `Accept` headers are not spec compliant, as they include MIME types supported by Chrome but not supported universally. While that's not the case, it required explanations as to why our current implementation is as it should be. I think a note in the spec would go a long way to prevent similar cases in the future.

This was [raised](https://github.com/whatwg/fetch/issues/43#issuecomment-98877690) in the past, but was concluded as unnecessary. I agree with past arguments that current spec language permits UAs to extend the `Accept` header, but stating it explicitly, saying that user agents may (or even should) extend the spec's values with MIME types they support, would be helpful.

/cc @igrigorik 

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