add 'comment' as optional field

[rlzijdeman]

Please consider adding 'comment' as an optional field. Such a field would give some space for elaboration or link to more extensive documentation.

[docuracy]

I agree, comments can sometimes be very useful and should not be disallowed or break validation, wherever they appear. There is a convention of using property names beginning with an underscore for 'private' properties, so perhaps _comment is the way to go?