Updating code format rules for documentation comments.

Making it clear that the star-every-line style of documentation comment makes avrodoc unhappy. Also putting in that we ought to keep to 80 characters per line, since I think we ought to keep to 80 characters per line.
Should we offer a guide on indentation of subsequent lines in a multi-line comment? (E.g. "should not be indented" or  "should be indented  two spaces" or "should be indented four spaces to line up with the first character of the first line")