Review comments on PDF21

The PDF Technology Notes link doesn't work. I think the href needs to be
"pdf.html#pdf_notes".

Change alt text on second image in Example 3 from "Tags panel showing first
list item correctly formatted nested under list tags." to "Tags panel
showing first list item correctly nested under list tags."

Modify the test procedure so that it says "For a list in a PDF document"
instead of "For each list in a PDF document". This should make it more
consistent with other test procedures for situations where there are
multiple techniques available.

There are what look like references to all the technology specs at the end
of the PDF Techniques document (which is where I am reviewing). Is that
expected?