Instructions to customTypes in the CMS

Im looking for a way of adding an instruction to the CMS for custom types. I see that some of the standard inputs already got these -  whats the name of the key i should use?

For instance the code below produces a text input with a label 'Project name'. I would like to also display a note or instruction below the text field in the CMS giving some additional instrcutions, like 'The project name can be no longer than 30 characters'.

  "customTypes" : [
    {
      "title" : "Project",
      "name" : "project",
      "base" : "text",
      "fields" : [
        {
          "name": "name",
          "title": "Project name",
          "type": "text"
        }]
	}]