The widget comes in two files HelpMain.lua and help.lua.
Help lua is the one you need to modify. See it's source code for documentation on this topic.

You can use "/luaui redraw" to redraw your currently opened page.

Syntax is very strict. All attributes are required. For example:
Correct:
<title=title>
Wrong:
<title = title>
<title="title">
<title=title />

These tags are supported:
<title=title>
<link=address>text</link> - equivalent to <a href> 
<image=image.png align=left|right|center width=pixels height=pixels>
<heading=fontsize> - equivalent to <h1>, <h2> ...
<li>
<include=page>