Missing documentation about creating a topic handler pointing to a nonexistent topic

I have, recently, discovered that when you create a handler to a nonexistent topic, the topic is created automatically. However, I couldn't find anything in the documentation about this behavior to confirm whether this is really the case or not.

I know it says a **topic is created on demand**, but it only implies that the topic will be created **once a task triggers an alert**, so it will be good to also document the behavior of creating a handler subscribed to a topic that doesn't yet exist.

Start by locating the existing documentation that says a topic is created on demand. Clarify there whether creating a handler for a nonexistent topic creates the topic immediately, and make the documentation explicit about this behavior. Done when the documentation answers the reported question without relying on implication.

Written by the indexing model from the issue text.