Publisher placeholder strings are not documented properly.

[borna-mdn]

Context
Publisher data string where user can define the message string that is published via MQTT/HTTP.

Issue
There is no way for the user to know what is a proper placeholder string to put in the data string section.

Example
User wants to embed temperature value in the data string. He needs to put $temperature placeholder in the data string. This is fine, and it is documented in an info dialog of the data string. But what happens if user wants to embed other values, like accelerometer data. Should he put $accelerometerx or $accelerometer_x in the data string. The list of supported placeholders is missing. User should have a way to retrieve the full list of supported placeholders.