It's generally the same markup used in button labels and evaluator columns.
This thread has the best list of things it can do:
The markup functionality was made mainly for internal use, and we didn't think people would use it much in buttons/columns, but it is turning out to be more useful than we expected, and was requested for use in dialogs as well. It was an "unofficial" feature, but is used enough now that it's a supported thing.
Making some proper docs for it is on our list, although not a top priority right now. The first few posts in the thread above cover most of the details.