dcache:improve documentation for kafka properties

Motivation

we have noticed a serious performence issue when kafka broker was done or nt available.

to fix this max.block.ms should be set to a lower value than the default wich is 60 sec.

the more files is going to be transfered the lower should be the number.

Acked-by: Tigran
Target: master, 8.0, 7.2, 7.1 7.0, 6.2
Require-book: yes

skel/share/defaults/kafka.properties

@@ -36,6 +36,15 @@ dcache.kafka.topic = billing
 # You can set the property value for other services with the  help of prefix, which is explained in the next section.
 # It is important to understand that Kafka Producer takes  MILLISECONDS as value.
 
+# If metadata is not available Kafka Producer is designed in a way that it blocks send() method for
+# up to max.block.ms, which means that this method is not async
+# the proposal to improve this has been (KIP-286
+# https://cwiki.apache.org/confluence/display/KAFKA/KIP-286%3A+producer.send%28%29+should+not+block+on+metadata+update )
+# dropped stating that ; `... the benefit of not having to wait for metadata is probably not worth the complexity added in producer.`
+# So it is strongly recommended in case if there is no a reliable Kafka Cluster or you have only one Broker
+# the  max.block.ms should be set to lower numbers less than 60 000 (default) is recommended, when you have a use case of big number of
+# files being Transferred. If you want complete non-blocking producer.send() can set max.block.ms to 0.
+
 dcache.kafka.maximum-block = 60
 
 (one-of?MILLISECONDS|SECONDS|MINUTES|HOURS|DAYS)\