If you open the url http://docs.sequelizejs.com/en/latest/docs/models-definition/ you can see that:

// Comments can be specified for each field for MySQL and PG
hasComment: { type: Sequelize.INTEGER, comment: "I'm a comment!" },

But this feature was not implemented in postgres dialect. I tried to add this in this PR #4385 but tests was failed. Somebody can fix it?