docs: clarified the relation between retention period and table period (#1129)

docs/configuration/README.md

@@ -745,7 +745,8 @@ and how to provision tables when DynamoDB is used as the backing store.
 [retention_deletes_enabled: <boolean> | default = false]
 
 # How far back tables will be kept before they are deleted. 0s disables
-# deletion.
+# deletion. The retention period must be a multiple of the index / chunks
+# table "period" (see period_config).
 [retention_period: <duration> | default = 0s]
 
 # Period with which the table manager will poll for tables.

docs/operations/storage/retention.md

@@ -10,8 +10,10 @@ Alternatively, the `table-manager.retention-period` and
 provided retention period needs to be a duration represented as a string that
 can be parsed using Go's [time.Duration](https://golang.org/pkg/time/#ParseDuration).
 
-> **WARNING**: The retention period should be at least twice the [duration of
-the periodic table config](https://github.com/grafana/loki/blob/347a3e18f4976d799d51a26cee229efbc27ef6c9/production/helm/loki/values.yaml#L53), which currently defaults to 7 days.
+> **WARNING**: The retention period must be a multiple of the index and chunks table
+`period`, configured in the [`period_config`](../../configuration/README.md#period_config)
+block. See the [Table Manager](./table-manager.md#retention) documentation for
+more information.
 
 When using S3 or GCS, the bucket storing the chunks needs to have the expiry
 policy set correctly. For more details check