Sidebar comments panel does not have a Help landing page. The dialog with the widgets is sw/uiconfig/swriter/ui/commentspanel.ui and sw/uiconfig/swriter/ui/commentsthread.ui helpID should be <bookmark branch="hid/modules/swriter/ui/commentspanel/<widget> ..."
The help pages should go to /swriter/01/
Dione Maddern committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/commit/e2903bddbdf7d9a4b10aba32a30b48d70e2feb4e tdf#163053 Add Help page for Comments sidebar panel in Writer