bigtable: update automated backups documentation for bigtable table (GoogleCloudPlatform#13326)

Author: ma-g-22

mmv1/third_party/terraform/website/docs/r/bigtable_table.html.markdown

@@ -100,7 +100,7 @@ to delete/recreate the entire `google_bigtable_table` resource.
 
 * `change_stream_retention` - (Optional) Duration to retain change stream data for the table. Set to 0 to disable. Must be between 1 and 7 days.
 
-* `automated_backup_policy` - (Optional) Defines an automated backup policy for a table, specified by Retention Period and Frequency. To disable, set both Retention Period and Frequency to 0.
+* `automated_backup_policy` - (Optional) Defines an automated backup policy for a table, specified by Retention Period and Frequency. To _create_ a table with automated backup disabled, omit this argument. To disable automated backup on an _existing_ table that has automated backup enabled, set both Retention Period and Frequency to 0.
 
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