[w3c/browser-payment-api] Editorial: Add missing xrefs to various specs (#400)

The spec currently relies on, but does not link to, terms in other specs (e.g., "queue a task"). We need to link those... To do list: 

 * [ ] DOM4
 * [ ] HTML
 * [ ] ECMA-262
 * [ ] Writing Promise-Using Specifications
 * [ ] Web IDL


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https://github.com/w3c/browser-payment-api/issues/400