shadow-dom: Refine a test for document's DOM tree accessors.

This change updates an existing test that checks whether document's DOM tree
accessors return nodes in a shadow tree or not. This change also adds
an extensive amount of comments that explain which specification each accessor
belongs to.

The test file was renamed to dom-tree-accessors-001.html.
Merge pull request #223 from yutak/shadow-dom/dom-tree-accessors

shadow-dom: Refine a test for document's DOM tree accessors.