[DOM4] Mutation algorithm imposed order on document children

I'm working on places where Webkit doesn't follow the DOM4 mutation
algorithm and one of the bugs is not throwing an exception when a doctype
node is inserted after an element in a document (or other permutations of
the same situation).

https://bugs.webkit.org/show_bug.cgi?id=88682
http://www.w3.org/TR/domcore/#mutation-algorithms

After discussing this with some other contributors there were questions on
why we're enforcing the order of the document child nodes. Specifically
since inserting a doctype node doesn't actually change the doctype so this
situation is very unlikely (possibly never happens) in the wild. Not
implementing this keeps the code simpler for a case that developers likely
never see.

Can we leave the behavior when your document is out of order unspecified?

- Elliott