[dom] Some tweaks for Nodes (#127)

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ensure pre-insertion validity (https://domspecwhatwgorg/#concept-node-ensure-pre-insertion-validity)
6 doctype
 "parent has a doctype child, an element is preceding child, or child is null and parent has an element child" << "preceding" operates on two object (nodes) but here `child` can be `null` so it's strange Maybe write ",child is not null and element is preceding child,"    
And mabye clarify some things in this three condition:
"doctype is following child" << we don't see what exactly "doctype" is, so better use "doctype in/from parent"
"element is preceding child" << like above, better use "element in/from parent"

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https://domspecwhatwgorg/#dom-node-normalize
One typo, in greenbox above we have ()()




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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/issues/127