Adds documentation

website/docs/r/elasticache_global_replication_group.html.markdown

@@ -44,13 +44,65 @@ resource "aws_elasticache_replication_group" "secondary" {
 }
 ```
 
+### Managing Redis Engine Versions
+
+The initial Redis version is deterined by the version set on the primary replication group.
+However, once it is part of a Global Replication Group,
+the Global Replication Group manages the version of all member replication groups.
+
+In this example,
+the primary replication group will be created with Redis 6.0,
+and then upgraded to Redis 6.2 once added to the Global Replication Group.
+The secondary replication group will be created with Redis 6.2.
+
+```terraform
+resource "aws_elasticache_global_replication_group" "example" {
+  global_replication_group_id_suffix = "example"
+  primary_replication_group_id       = aws_elasticache_replication_group.primary.id
+
+  engine_version = "6.2"
+}
+
+resource "aws_elasticache_replication_group" "primary" {
+  replication_group_id          = "example-primary"
+  replication_group_description = "primary replication group"
+
+  engine         = "redis"
+  engine_version = "6.0"
+  node_type      = "cache.m5.large"
+
+  number_cache_clusters = 1
+}
+
+resource "aws_elasticache_replication_group" "secondary" {
+  provider = aws.other_region
+
+  replication_group_id          = "example-secondary"
+  replication_group_description = "secondary replication group"
+  global_replication_group_id   = aws_elasticache_global_replication_group.example.global_replication_group_id
+
+  number_cache_clusters = 1
+}
+```
+
 ## Argument Reference
 
 The following arguments are supported:
 
+* `engine_version` - (Optional) Redis version to use for the Global Replication Group.
+  When creating, by default the Global Replication Group inherits the version of the primary replication group.
+  If a version is specified, the Global Replication Group and all member replication groups will be upgraded to this version.
+  Cannot be downgraded without replacing the Global Replication Group and all member replication groups.
+  If the version is 6 or higher, the major and minor version can be set, e.g., `6.2`,
+  or the minor version can be unspecified which will use the latest version at creation time, e.g., `6.x`.
+  The actual engine version used is returned in the attribute `engine_version_actual`, see [Attributes Reference](#attributes-reference) below.
 * `global_replication_group_id_suffix` – (Required) The suffix name of a Global Datastore. If `global_replication_group_id_suffix` is changed, creates a new resource.
 * `primary_replication_group_id` – (Required) The ID of the primary cluster that accepts writes and will replicate updates to the secondary cluster. If `primary_replication_group_id` is changed, creates a new resource.
 * `global_replication_group_description` – (Optional) A user-created description for the global replication group.
+* `parameter_group_name` - (Optional) An ElastiCache Parameter Group to use for the Global Replication Group.
+  Required when upgrading a major engine version, but will be ignored if left configured after the upgrade is complete.
+  Specifying without a major version upgrade will fail.
+  Note that ElastiCache creates a copy of this parameter group for each member replication group.
 
 ## Attributes Reference