add note for supported units of min_index_age in ISM policies document (

opensearch-project#1600)

Signed-off-by: syrinx05p <[email protected]>

Signed-off-by: syrinx05p <[email protected]>

_im-plugin/ism/policies.md

@@ -267,7 +267,7 @@ 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. See **Important** note above. | `string` | `20gb` or `5mb` | No
 `min_primary_shard_size` | The minimum storage size of a **single primary shard** required to roll over the index. For example, if you set `min_primary_shard_size` to 30 GiB and **one of** the primary shards in the index has a size greater than the condition, the rollover occurs. See **Important** note above. | `string` | `20gb` or `5mb` | No
 `min_doc_count` |  The minimum number of documents required to roll over the index. See **Important** note above. | `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. See **Important** note above. | `string` | `5d` or `7h` | No
+`min_index_age` |  The minimum age required to roll over the index. Index age is the time between its creation and the present. Supported units are `d` (days), `h` (hours), `m` (minutes), `s` (seconds), `ms` (milliseconds), and `micros` (microseconds). See **Important** note above. | `string` | `5d` or `7h` | No
 
 ```json
 {