docs: document retention,grace period for window queries

docs-md/concepts/time-and-windows-in-ksqldb-queries.md

@@ -365,4 +365,42 @@ SELECT o.order_id, o.total_amount, o.customer_name, s.shipment_id, s.warehouse
 For more information on joins, see
 [Join Event Streams with ksqlDB](../developer-guide/joins/join-streams-and-tables.md).
 
+### Late Arriving Events
+
+Often times, events that belong to a window can arrive late (e.g slow networks) and a grace period 
+may be required to ensure the events are accepted into the window. ksqlDB allows configuring this
+behavior, for each of the window types above. 
+
+For example, to allow events to be accepted upto 2 hours of delay after the window ends, 
+you might run a query like:
+
+```sql
+SELECT orderzip_code, TOPK(order_total, 5) FROM orders
+  WINDOW TUMBLING (SIZE 1 HOUR, GRACE PERIOD 2 HOURS) 
+  GROUP BY order_zipcode
+  EMIT CHANGES;
+```
+
+Events that arrive later than the grace period, are dropped and not included in the aggregate result.
+
+### Window Retention 
+
+ksqlDB supports configuring how many windows in the past to retain for pull and push queries for 
+each window type. This capability can be very useful for interactive applications that use ksqlDB 
+as their primary serving data store.
+
+For example, to retain the computed windowed aggregation results for a week, 
+you might run the following query:
+
+```sql
+SELECT regionid, COUNT(*) FROM pageviews
+  WINDOW HOPPING (SIZE 30 SECONDS, ADVANCE BY 10 SECONDS, RETENTION 7 DAYS, GRACE PERIOD 30 MINUTES)
+  WHERE UCASE(gender)='FEMALE' AND LCASE (regionid) LIKE '%_6'
+  GROUP BY regionid
+  EMIT CHANGES;
+```
+
+Note that the specified retention period should be larger than the sum of window size and any grace
+period.
+
 Page last revised on: {{ git_revision_date }}

ksqldb-streams/src/test/java/io/confluent/ksql/execution/streams/MaterializedFactoryTest.java

@@ -76,8 +76,7 @@ public void shouldSetupRetentionWhenNonEmpty() {
     when(retention.get()).thenReturn(Duration.ofSeconds(10));
 
     // When:
-    final Materialized<String, GenericRow, StateStore> returned
-        = MaterializedFactory.create().create(keySerde, rowSerde, OP_NAME, retention);
+    MaterializedFactory.create().create(keySerde, rowSerde, OP_NAME, retention);
 
     // Then:
     verify(retention).isPresent();