Improve formatting of symbol descriptions

[Mark Wubben]

Symbol descriptions are currently formatted as-is. If they contain linebreaks they'll disrupt the formatting output. Invisible or ambiguous characters aren't escaped either.

At the very least characters should be escaped, like #4. Perhaps linebreaks should be preserved in the output (it ought to be technically possible), but it's probably simpler to just escape them. Note that when used as a key a symbol shouldn't span multiple lines.