Add descriptions to MergeRequestStateEnum

Add missing values
Add descriptions to values
Update schema
Update documentation

app/graphql/types/merge_request_state_enum.rb

@@ -5,6 +5,6 @@ module Types
     graphql_name 'MergeRequestState'
     description 'State of a GitLab merge request'
 
-    value 'merged'
+    value 'merged', description: "Merge Request has been merged"
   end
 end

doc/api/graphql/reference/gitlab_schema.graphql

@@ -15696,6 +15696,10 @@ enum MergeRequestState {
   all
   closed
   locked
+
+  """
+  Merge Request has been merged
+  """
   merged
   opened
 }

doc/api/graphql/reference/gitlab_schema.json

@@ -43076,7 +43076,7 @@
             },
             {
               "name": "merged",
-              "description": null,
+              "description": "Merge Request has been merged",
               "isDeprecated": false,
               "deprecationReason": null
             }

doc/api/graphql/reference/index.md

@@ -4991,7 +4991,7 @@ State of a GitLab merge request.
 | `all` |  |
 | `closed` |  |
 | `locked` |  |
-| `merged` |  |
+| `merged` | Merge Request has been merged |
 | `opened` |  |
 
 ### MilestoneStateEnum