Add documenation about dynamic config keys and the need to disambigua…

…te them in the config file or as system properties. Also explain the need to use double underscore for properties that include a period in their key

site/documentation/content/admin-guide/hono-kafka-client-configuration.md

@@ -111,6 +111,22 @@ The Kafka clients require at least the `bootstrap.servers` property to be set. T
 required to enable Kafka based messaging.
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+The Kafka client configuration is composed of a number of properties that have an ambiguous mapping to environment
+variable names. For example, the property keys `a.b`, `a-b` and `a_b` would all map to the environment variable `A_B`.
+To resolve this ambiguity, the configuration framework relies on the correct property being defined in a configuration
+source with a lower presedence than the environment variable configuration source. It's therefore necessary to provide
+any such configuration property in the `application.yml` file or as a Java system property to help the configuration
+system disambiguate the property key, even if only providing a placeholder value. Once that has been done, it's 
+possible to override the value provided with an environment variable.
+{{% /notice %}}
+
+{{% notice tip %}}
+Any of the Kafka configuration options that use a period (`.`) in the key must use a double underscore in the 
+environment variable name. For example, the environment variable name to represent 
+`hono.kafka.commonClientConfig."bootstrap.servers"` is `HONO_KAFKA_COMMONCLIENTCONFIG__BOOTSTRAP_SERVERS__`.
+{{% /notice %}}
+
 ## Using TLS
 
 The factory can be configured to use TLS for authenticating the brokers in the Kafka cluster during connection