[dom] "If context object’s parent is parent, re place t..."

annevk has just created a new issue for https://github.com/whatwg/dom:

== "If context object’s parent is parent, replace t..." ==
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.

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