Added information about kafka log retention policies (#2052)

* Added info about kafka log retention policies

* Update docs/home/howto/RETENTION.md

Co-authored-by: to-bar <46519524+to-bar@users.noreply.github.com>

* Update docs/home/howto/RETENTION.md

Co-authored-by: to-bar <46519524+to-bar@users.noreply.github.com>

* Update docs/home/howto/RETENTION.md

Co-authored-by: to-bar <46519524+to-bar@users.noreply.github.com>

Co-authored-by: atsikham <atsikham@users.noreply.github.com>
Co-authored-by: to-bar <46519524+to-bar@users.noreply.github.com>

docs/home/howto/RETENTION.md

@@ -1,4 +1,5 @@
-An Epiphany cluster has a number of components which log, collect and retain data. To make sure that these do not exceed the usable storage of the machines there running on the following configurations are available.
+An Epiphany cluster has a number of components which log, collect and retain data. To make sure that these do not exceed
+the usable storage of the machines they running on, the following configurations are available.
 
 ## Elasticsearch
 
@@ -10,7 +11,44 @@ TODO
 
 ## Kafka
 
-TODO
+There are two types of retention policies that can be configured at the broker or topic levels: based on time or size.
+Epiphany defines the same default value for broker size retention policy as Kafka, -1, which means that no
+size limit is applied.
+
+To define new log retention values following configuration can be used:
+
+```yaml
+kind: configuration/kafka
+title: "Kafka"
+name: default
+specification:
+    kafka_var:
+        partitions: 8
+        log_retention_hours: 168
+        log_retention_bytes: -1
+```
+
+### Configuration parameters
+
+#### specification.kafka_var.partitions
+
+Sets [num.partitions](https://kafka.apache.org/documentation/#brokerconfigs_num.partitions) parameter
+
+#### specification.kafka_var.log_retention_hours
+
+Sets [log.retention.hours](https://kafka.apache.org/documentation/#brokerconfigs_log.retention.bytes) parameter
+
+#### specification.kafka_var.log_retention_bytes
+
+Sets [log.retention.bytes](https://kafka.apache.org/documentation/#brokerconfigs_log.retention.bytes) parameter
+
+---
+**NOTE**
+
+Since this limit is enforced at the partition level, multiply it by the number of partitions to compute the topic
+retention in bytes.
+
+---
 
 ## Kibana