docs: add variable attribute

Signed-off-by: aevesdocker <allie.sadler@docker.com>

docs/bake-reference.md

@@ -1099,6 +1099,7 @@ or interpolate them in attribute values in your Bake file.
 variable "TAG" {
   type = string
   default = "latest"
+  description: "Tag to use for build"
 }
 
 target "webapp-dev" {
@@ -1111,6 +1112,8 @@ You can assign a default value for a variable in the Bake file,
 or assign a `null` value to it. If you assign a `null` value,
 Buildx uses the default value from the Dockerfile instead.
 
+You can also add a description of the variables purpose with the `description` field. This attribute is useful when combined with the `docker buildx bake --list=variables` option, providing a more informative output when listing the available variables in a Bake file.
+
 You can override variable defaults set in the Bake file using environment variables.
 The following example sets the `TAG` variable to `dev`,
 overriding the default `latest` value shown in the previous example.