reintroduce 'misleading' comment

The last version of gloss-export removes a comment which explains that the size of the canvas is limited, claiming that the comment is "misleading". However, the comment was correct: it is still not possible (at least on my machine), to generate files with beyond a certain size. The particular limit for my machine did not match the limit mentioned in the comment, but did perfectly match the size of my screen (minus the menu bar).