Including formatting the search description is not supported. I mentioned your post to the WebLink team so they can take a look, but note that the description was intended as a short amount of unformatted text with the expectation that instructions etc. would be contained within the search form itself.

From a post on the old forums:

...you cannot format the text for the description, and this includes line breaks.

If you find that the search form is not at neat as you would like, you could try using a simple table to section the search form area into columns and/or rows. For example:

That was created by using a 3 column, 2 row table, with line breaks at the end of each row for extra spacing.