doc: Define changelog entries for ISS feature and new attributes (#2478)

* include changelog entries

* address review comments

.changelog/2420.txt

@@ -1,11 +1,11 @@
 ```release-note:note
-resource/mongodbatlas_advanced_cluster: Deprecates following attributes - `replication_specs.#.id`, `replication_specs.#.num_shards`, `disk_size_gb`, `advanced_configuration.0.default_read_concern`, and  `advanced_configuration.0.fail_index_key_too_long`.
+resource/mongodbatlas_advanced_cluster: Deprecates `replication_specs.#.id`, `replication_specs.#.num_shards`, `disk_size_gb`, `advanced_configuration.0.default_read_concern`, and  `advanced_configuration.0.fail_index_key_too_long`. To learn more, see the [1.18.0 Migration Guide](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/guides/1.18.0-upgrade-guide).
 ```
 
 ```release-note:note
-data-source/mongodbatlas_advanced_cluster: Deprecates following attributes - `replication_specs.#.id`, `replication_specs.#.num_shards`, `disk_size_gb`, `advanced_configuration.0.default_read_concern`, and  `advanced_configuration.0.fail_index_key_too_long`.
+data-source/mongodbatlas_advanced_cluster: Deprecates `replication_specs.#.id`, `replication_specs.#.num_shards`, `disk_size_gb`, `advanced_configuration.0.default_read_concern`, and  `advanced_configuration.0.fail_index_key_too_long`. To learn more, see the [1.18.0 Migration Guide](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/guides/1.18.0-upgrade-guide).
 ```
 
 ```release-note:note
-data-source/mongodbatlas_advanced_clusters:Deprecates following attributes - `replication_specs.#.id`, `replication_specs.#.num_shards`, `disk_size_gb`, `advanced_configuration.0.default_read_concern`, and  `advanced_configuration.0.fail_index_key_too_long`.
+data-source/mongodbatlas_advanced_clusters: Deprecates `replication_specs.#.id`, `replication_specs.#.num_shards`, `disk_size_gb`, `advanced_configuration.0.default_read_concern`, and  `advanced_configuration.0.fail_index_key_too_long`. To learn more, see the [1.18.0 Migration Guide](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/guides/1.18.0-upgrade-guide).
 ```

.changelog/2478.txt

@@ -0,0 +1,19 @@
+```release-note:enhancement
+resource/mongodbatlas_advanced_cluster: Supports defining cluster shards with independent `replication_specs` objects. This feature enables defining independent scaled shards. To learn more, see the [Advanced Cluster New Sharding Schema Migration Guide](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/guides/advanced-cluster-new-sharding-schema).
+```
+
+```release-note:note
+resource/mongodbatlas_advanced_cluster: Using this new version impacts the possibility of editing the definition of multi shard clusters in the Atlas UI. This impact is limited to the first weeks of August.
+```
+
+```release-note:enhancement
+resource/mongodbatlas_advanced_cluster: Adds `replication_specs.*.zone_id` and `replication_specs.*.region_configs.*.(electable_specs|analytics_specs|read_only_specs).disk_size_gb` attributes. To learn more, see the [1.18.0 Migration Guide](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/guides/1.18.0-upgrade-guide) and resource documentation.
+```
+
+```release-note:enhancement
+data-source/mongodbatlas_advanced_cluster: Adds `use_replication_spec_per_shard`, `replication_specs.*.zone_id`, and `replication_specs.*.region_configs.*.(electable_specs|analytics_specs|read_only_specs).disk_size_gb` attributes. To learn more, see the [1.18.0 Migration Guide](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/guides/1.18.0-upgrade-guide) and data source documentation.
+```
+
+```release-note:enhancement
+data-source/mongodbatlas_advanced_clusters: Adds `use_replication_spec_per_shard`, `replication_specs.*.zone_id`, and `replication_specs.*.region_configs.*.(electable_specs|analytics_specs|read_only_specs).disk_size_gb` attributes. To learn more, see the [1.18.0 Migration Guide](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/guides/1.18.0-upgrade-guide) and data source documentation.
+```

docs/guides/1.18.0-upgrade-guide.md

@@ -8,8 +8,8 @@ The Terraform MongoDB Atlas Provider version 1.18.0 has a number of new and exci
 
 **New Resources, Data Sources, and Features:**
 
-- You can now scale the instance size and disk IOPS independently for each individual shard for sharded and geo-sharded clusters defined with `mongodbatlas_advanced_cluster`. To learn more, see the [Migration Guide](advanced-cluster-new-sharding-schema). As part of these changes two new attributes are added:
-    - Use the `replication_specs.*.zone_id` attribute in the `mongodbatlas_advanced_cluster` resource and data sources to specify the zone of each `replication_specs` object.
+- You can now scale the instance size and disk IOPS independently for each individual shard for sharded and geo-sharded clusters defined with `mongodbatlas_advanced_cluster`. To learn more, see the [Advanced Cluster New Sharding Schema Migration Guide](advanced-cluster-new-sharding-schema). As part of these changes two new attributes are added:
+    - Use the `replication_specs.*.zone_id` attribute in the `mongodbatlas_advanced_cluster` resource and data sources to identify the zone of each `replication_specs` object.
   - Use the `use_replication_spec_per_shard` attribute in the `mongodbatlas_advanced_cluster` data sources to specify whether to obtain `replication_specs` objects for each shard.
 
 - The cloud backup schedule for a cluster can now be used to configure copy settings by zone instead of replication object in each zone with the `mongodbatlas_cloud_backup_schedule` resource and data sources.  This feature also enables you to reference cluster zones using independent shard scaling, which no longer supports `replication_spec.*.id`.