resource/aws_ssm_document: Support in-place version_name updates, add CHANGELOG entry for #14128

Output from acceptance testing in AWS Commercial:

```
--- PASS: TestAccAWSSSMDocument_params (30.50s)
--- PASS: TestAccAWSSSMDocument_basic (32.24s)
--- PASS: TestAccAWSSSMDocument_session (32.29s)
--- PASS: TestAccAWSSSMDocument_permission_private (32.51s)
--- PASS: TestAccAWSSSMDocument_permission_batching (32.78s)
--- PASS: TestAccAWSSSMDocument_permission_public (32.80s)
--- PASS: TestAccAWSSSMDocument_automation (37.50s)
--- PASS: TestAccAWSSSMDocument_VersionName (46.97s)
--- PASS: TestAccAWSSSMDocument_DocumentFormat_YAML (46.99s)
--- PASS: TestAccAWSSSMDocument_target_type (48.06s)
--- PASS: TestAccAWSSSMDocument_update (48.17s)
--- PASS: TestAccAWSSSMDocument_SchemaVersion_1 (48.20s)
--- PASS: TestAccAWSSSMDocument_Tags (57.02s)
--- PASS: TestAccAWSSSMDocument_permission_change (57.33s)
--- PASS: TestAccAWSSSMDocument_package (73.74s)
```

Output from acceptance testing in AWS GovCloud (US):

```
--- PASS: TestAccAWSSSMDocument_basic (34.86s)
--- PASS: TestAccAWSSSMDocument_session (39.41s)
--- PASS: TestAccAWSSSMDocument_params (39.84s)
--- PASS: TestAccAWSSSMDocument_permission_private (40.26s)
--- PASS: TestAccAWSSSMDocument_permission_public (40.37s)
--- PASS: TestAccAWSSSMDocument_permission_batching (40.94s)
--- PASS: TestAccAWSSSMDocument_automation (52.65s)
--- PASS: TestAccAWSSSMDocument_target_type (54.33s)
--- PASS: TestAccAWSSSMDocument_update (55.65s)
--- PASS: TestAccAWSSSMDocument_DocumentFormat_YAML (55.72s)
--- PASS: TestAccAWSSSMDocument_VersionName (55.79s)
--- PASS: TestAccAWSSSMDocument_SchemaVersion_1 (55.96s)
--- PASS: TestAccAWSSSMDocument_Tags (66.39s)
--- PASS: TestAccAWSSSMDocument_permission_change (68.28s)
--- PASS: TestAccAWSSSMDocument_package (80.15s)
```

Author: bflad

.changelog/14128.txt

@@ -0,0 +1,3 @@
+```release-note:enhancement
+resource/aws_ssm_document: Add `version_name` argument
+```

aws/resource_aws_ssm_document.go

@@ -175,8 +175,6 @@ func resourceAwsSsmDocument() *schema.Resource {
 			"version_name": {
 				Type:     schema.TypeString,
 				Optional: true,
-				Computed: true,
-				ForceNew: true,
 			},
 		},
 	}
@@ -658,6 +656,10 @@ func updateAwsSSMDocument(d *schema.ResourceData, meta interface{}) error {
 		updateDocInput.TargetType = aws.String(v.(string))
 	}
 
+	if v, ok := d.GetOk("version_name"); ok {
+		updateDocInput.VersionName = aws.String(v.(string))
+	}
+
 	if d.HasChange("attachments_source") {
 		updateDocInput.Attachments = expandSsmAttachmentsSources(d.Get("attachments_source").([]interface{}))
 	}

aws/resource_aws_ssm_document_test.go

@@ -30,6 +30,7 @@ func TestAccAWSSSMDocument_basic(t *testing.T) {
 					testAccCheckResourceAttrRfc3339(resourceName, "created_date"),
 					resource.TestCheckResourceAttr(resourceName, "tags.%", "0"),
 					resource.TestCheckResourceAttrSet(resourceName, "document_version"),
+					resource.TestCheckResourceAttr(resourceName, "version_name", ""),
 				),
 			},
 			{
@@ -72,7 +73,7 @@ func TestAccAWSSSMDocument_target_type(t *testing.T) {
 	})
 }
 
-func TestAccAWSSSMDocument_version_name(t *testing.T) {
+func TestAccAWSSSMDocument_VersionName(t *testing.T) {
 	name := acctest.RandString(10)
 	resourceName := "aws_ssm_document.test"
 	resource.ParallelTest(t, resource.TestCase{
@@ -659,14 +660,14 @@ DOC
 func testAccAWSSSMDocumentBasicConfigVersionName(rName, version string) string {
 	return fmt.Sprintf(`
 resource "aws_ssm_document" "test" {
-  name          = "%s"
+  name          = %[1]q
   document_type = "Command"
-  version_name  = "%s"
+  version_name  = %[2]q
 
   content = <<DOC
     {
        "schemaVersion": "2.0",
-       "description": "Sample version 2.0 document v2",
+       "description": "Sample version 2.0 document %[2]s",
        "parameters": {
 
        },

website/docs/r/ssm_document.html.markdown

@@ -54,8 +54,8 @@ The following arguments are supported:
 * `document_type` - (Required) The type of the document. Valid document types include: `Automation`, `Command`, `Package`, `Policy`, and `Session`
 * `permissions` - (Optional) Additional Permissions to attach to the document. See [Permissions](#permissions) below for details.
 * `target_type` - (Optional) The target type which defines the kinds of resources the document can run on. For example, /AWS::EC2::Instance. For a list of valid resource types, see AWS Resource Types Reference (http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-template-resource-type-ref.html)
-* `version_name` - (Optional) A field specifying the version of the artifact you are creating with the document. For example, "Release 12, Update 6"
 * `tags` - (Optional) A map of tags to assign to the object.
+* `version_name` - (Optional) A field specifying the version of the artifact you are creating with the document. For example, "Release 12, Update 6". This value is unique across all versions of a document and cannot be changed for an existing document version.
 
 ## attachments_source