We assume that comments are defined as parsed by the
docx reader:
I want <span class="comment-start" id="0" author="Jesse Rosenthal"
date="2016-05-09T16:13:00Z">I left a comment.</span>some text to
have a comment <span class="comment-end" id="0"></span>on it.
We assume also that the id attributes are unique and properly
matched between comment-start and comment-end.
Closes#2994.