[whatwg/dom] Address feedback from Travis and Sangwhan (#702)

There's some useful commentary in https://github.com/w3ctag/design-reviews/issues/229#issuecomment-424390065 not all of which is captured by other issues. In particular:

* Remove the outdated mailing list advice.
* Perhaps replace "useless" with "historical" (although the usage of it was meant to convey "has no use" it's apparently taken to mean useless' more informal derogatory meaning).
* Link "get the parent" issue.
* Explain why `attachShadow()` has the safelist it has.

(The IDL remarks don't have an issue, but they shouldn't be addressed at this level, as they affect all IDL standards.)

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