Putting asterisks around text makes it italic.
Starting a line with a hash makes is large and boldface.

Are there any other text variations possible? I think it would be great if these could be documented.

And another question: how do I make one and more line comments? How do I comment out a part of the source?

That's Markdown. The "Help" tab in the editor mentions that, and links to the docs: http://daringfireball.net/projects/markdown/syntax

Ah, thanks! I had missed that.

And comments?

There's no comment syntax currently, but you could use HTML comments within a section or passage <!-- like this -->

OK. I like the 'currently', because commenting out something that works is better than deleting it before trying to replace it by something hopefully better. ;-D