Support comment attribute

[typed-sigterm]

Though we can impl json_schema to achieve this now, I think it isn't a bad idea to support it by default :)

#[schemars(comment = "Do not modify since this is auto-generated.")]
pub struct SomeSchema {
}

↓

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "SomeSchema",
  "type": "object",
  "comment": "Do not modify since this is auto-generated."
}