If you only want to change the comment it does not need to be done in code just use

map-comment "example<br>example"

Real example at

http://www.cave-registry.org.uk/svn/CheddarCatchment/thconfig_A0plan

If you are making your own legend or doing something that requires tex, you need to use
\medskip

for a new line (think \bigskip also works)

again real example altering the \cavename at

http://www.cave-registry.org.uk/svn/CheddarCatchment/thconfigres+whitespot

The outcomes of the above (or maybe a little older version) can be seen at

http://cave-registry.org.uk/cheddarcatchment

Andrew