[docs] Add documentation for zero-cost async stack traces.

[bmeurer]

Bug: v8:7522
Ref: nodejs/node#11865
Design-Document: http://bit.ly/v8-zero-cost-async-stack-traces

[googlebot]

So there's good news and bad news.

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[googlebot]

A Googler has manually verified that the CLAs look good.

(Googler, please make sure the reason for overriding the CLA status is clearly documented in these comments.)

[mathiasbynens]

Thanks!