Update expired field description

app/graphql/types/milestone_type.rb

@@ -27,7 +27,7 @@ module Types
           description: 'State of the milestone.'
 
     field :expired, GraphQL::BOOLEAN_TYPE, null: false,
-          description: 'Expired state of the milestone (a milestone is expired when the due date is past the current date).'
+          description: 'Expired state of the milestone (a milestone is expired when the due date is past the current date). Defaults to `false` when due date has not been set.'
 
     field :web_path, GraphQL::STRING_TYPE, null: false, method: :milestone_path,
           description: 'Web path of the milestone.'

doc/api/graphql/reference/index.md

@@ -10813,7 +10813,7 @@ Represents a milestone.
 | <a id="milestonecreatedat"></a>`createdAt` | [`Time!`](#time) | Timestamp of milestone creation. |
 | <a id="milestonedescription"></a>`description` | [`String`](#string) | Description of the milestone. |
 | <a id="milestoneduedate"></a>`dueDate` | [`Time`](#time) | Timestamp of the milestone due date. |
-| <a id="milestoneexpired"></a>`expired` | [`Boolean!`](#boolean) | Expired state of the milestone (a milestone is expired when the due date is past the current date). |
+| <a id="milestoneexpired"></a>`expired` | [`Boolean!`](#boolean) | Expired state of the milestone (a milestone is expired when the due date is past the current date). Defaults to `false` when due date has not been set. |
 | <a id="milestonegroupmilestone"></a>`groupMilestone` | [`Boolean!`](#boolean) | Indicates if milestone is at group level. |
 | <a id="milestoneid"></a>`id` | [`ID!`](#id) | ID of the milestone. |
 | <a id="milestoneiid"></a>`iid` | [`ID!`](#id) | Internal ID of the milestone. |