The answer can be found in the DTD: http://www.knime.org/Node.dtd

For a short summary:

the description has to be within the "intro" tags.

elements can be used to create headers (h3, h4),

lists (ul = bullet point, il = numbered list) with li for list elements

"courier-like" text (tt)

super script (sup) or subscripts (sub)

bold (b). underlined (u), italic (i)

there can be also hyperlinks (using a href=...)