[dom] "If context object’s parent is parent, replace t..." (#48)

https://dom.spec.whatwg.org/#interface-childnode

> If context object’s parent is parent, replace the context object with node within parent. 

Add a note here

```
<p class=note><a>Context object</a> could have been inserted into <var>node</var>.
```
Once @tabatkins fixes bikeshed since currently DOM generates a bunch of errors that seem like regressions.

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https://github.com/whatwg/dom/issues/48