Bug 23196: (follow-up) Add comments to highlight structure

Bug 23196: (follow-up) Add comments to highlight structure

This follow-up adds a few comments to the markup in order to highlight
and clarify the the HTML structure.

To test, apply the patch and perform a diff against master using the
'-w' flag:

git diff -w origin/master

You should only see the addition of HTML comments.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit fc39e24eb502c1f724e8c5aa8b7264db901be278)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>