Merge pull request #85146 from robinsh/robinsh-0812-msgen

Add line explaining use limits.

Author: PRMerger6

articles/iot-hub/iot-hub-message-enrichments-overview.md

@@ -24,9 +24,9 @@ A message enrichment has three key elements:
 
 * One or more [endpoints](iot-hub-devguide-endpoints.md) for which the enrichment should be applied.
 
-The key can be any string.
+The **key** is a string. A key can only contain alphanumeric characters or these special characters: hyphen (`-`), underscore (`_`), and period (`.`).
 
-The value can be any of the following examples:
+The **value** can be any of the following examples:
 
 * Any static string. Dynamic values such as conditions, logic, operations, and functions are not allowed. For example, if you develop a SaaS application that is used by several customers, you can assign an identifier to each customer and make that identifier available in the application. When the application runs, IoT Hub will stamp the device telemetry messages with the customer's identifier, making it possible to process the messages differently for each customer.