doc(rdb): read replica: add same_zone field to the documentation (#2231)

Author: Mia-Cross

docs/resources/rdb_read_replica.md

@@ -70,6 +70,8 @@ The following arguments are supported:
       CIDR notation. The IP network address within the private subnet is determined by the IP Address Management (IPAM)
       service if not set.
 
+- `same_zone` - (Defaults to `false`) Defines whether to create the replica in the same availability zone as the main instance nodes or not.
+
 - `region` - (Defaults to [provider](../index.md#region) `region`) The [region](../guides/regions_and_zones.md#regions)
   in which the Database read replica should be created.