[dom] clarify insert mutation algo step 6.1 (#173)

Step 6.1 of https://dom.spec.whatwg.org/#concept-node-insert is a bit unclear because it does not reference the **insert** and **append** mutation algorithms. This PR adds the links.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/173

-- Commit Summary --

  * clarify insert mutation algo step 6.1

-- File Changes --

    M dom.bs (4)

-- Patch Links --

https://github.com/whatwg/dom/pull/173.patch
https://github.com/whatwg/dom/pull/173.diff

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https://github.com/whatwg/dom/pull/173