adding a label to a Content Slice in the document mask

page.json

{
  "Content" : {
    "body" : {
      "fieldset" : "Dynamic page zone...",
      "type" : "Slices",
      "config" : {
        "labels" : {
          "featured-items" : [ {
            "name" : "full",
            "display" : "Full featured"
          }, {
            "name" : "mini",
            "display" : "Mini"
          }, {
            "name" : "preview",
            "display" : "With preview"
          }, {
            "name" : "simple",
            "display" : "Simple"
          }, {
            "name" : "largeblock",
            "display" : "Large block"
          } ]
        },
        "choices" : {
          "featured-items" : {
            "type" : "Group",
            "fieldset" : "Featured items",
            "config" : {
              "fields" : {
                "illustration" : {
                  "type" : "Image",
                  "config" : {
                    "constraint" : {
                      "height" : 1200
                    },
                    "thumbnails" : [ {
                      "name" : "Icon",
                      "width" : 300,
                      "height" : 300
                    } ]
                  }
                },
                "title" : {
                  "type" : "StructuredText",
                  "config" : {
                    "single" : "heading3",
                    "placeholder" : "item title..."
                  }
                },
                "summary" : {
                  "type" : "StructuredText",
                  "fieldset" : "Short Summary",
                  "config" : {
                    "placeholder" : "item summary...",
                    "single" : "paragraph"
                  }
                },
                "read-more" : {
                  "type" : "Link",
                  "fieldset" : "Link to read more",
                  "config" : {
                    "select" : "document",
                    "masks" : [ "page" ],
                    "placeholder" : "Select a Read More page"
                  }
                },
                "read-more-label" : {
                  "type" : "StructuredText",
                  "config" : {
                    "placeholder" : "Read more label"
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}