Add doc on action rollover min_primary_shard_size

Reference https://github.com/opensearch-project/index-management/pull/220

Signed-off-by: Stein Arne Storslett <sastorsl@users.noreply.github.com>

_im-plugin/ism/policies.md

@@ -206,6 +206,7 @@ Set `index.plugins.index_state_management.rollover_alias` as the alias to rollov
 Parameter | Description | Type | Example | Required
 :--- | :--- |:--- |:--- |
 `min_size` | The minimum size of the total primary shard storage (not counting replicas) required to roll over the index. For example, if you set `min_size` to 100 GiB and your index has 5 primary shards and 5 replica shards of 20 GiB each, the total size of all primary shards is 100 GiB, so the rollover occurs. ISM doesn't check indexes continually, so it doesn't roll over indexes at exactly 100 GiB. Instead, if an index is continuously growing, ISM might check it at 99 GiB, not perform the rollover, check again when the shards reach 105 GiB, and then perform the operation. | `string` | `20gb` or `5mb` | No
+`min_primary_shard_size` | The minimum size of a *single primary shard* storage (not counting replicas) required to roll over the index. For example, if you set `min_primary_shard_size` to 30 GiB and there is a *single primary shard* that has a size greater than the condition, the rollover occurs. ISM doesn't check indexes continually, so it doesn't roll over indexes at exactly 30 GiB. Instead, if an index shard is continuously growing, ISM might check it at 29 GiB, not perform the rollover, check again when the shards reach 32 GiB, and then perform the operation. | `string` | `20gb` or `5mb` | No
 `min_doc_count` |  The minimum number of documents required to roll over the index. | `number` | `2000000` | No
 `min_index_age` |  The minimum age required to roll over the index. Index age is the time between its creation and the present. | `string` | `5d` or `7h` | No
 
@@ -217,6 +218,14 @@ Parameter | Description | Type | Example | Required
 }
 ```
 
+```json
+{
+  "rollover": {
+    "min_primary_shard_size": "30gb"
+  }
+}
+```
+
 ```json
 {
   "rollover": {
@@ -618,7 +627,8 @@ After 30 days, the policy moves this index into a `delete` state. The service se
         "actions": [
           {
             "rollover": {
-              "min_index_age": "1d"
+              "min_index_age": "1d",
+              "min_primary_shard_size": "30gb"
             }
           }
         ],
@@ -666,7 +676,11 @@ After 30 days, the policy moves this index into a `delete` state. The service se
           }
         ]
       }
-    ]
+    ],
+    "ism_template": {
+      "index_patterns": ["log*"],
+      "priority": 100
+    }
   }
 }
 ```