Clarify correlation between Event Processors and Databases

[smcvb]

Now and then users may reach issues with their Axon application that, for example, the database connection pool gets exhausted.
The thing that's not immediately evident is the number of threads Axon might create, depending on the number of Event Processors in place.

Although I'm not sold on the exact format yet, I think a (small?) section describing some pointers about the correlation between Event Processors and Databases would be beneficial.
I'd expect such a section to describe in short:

• The fact Streaming Processors use different threads.
• The fact these processor threads open a connection with the Event Store storage solution.
• The fact these processor threads open a connection with the Token Store storage solution.
• The fact these processor threads will start any operation the user's drafted up in Event Handling Components.

[smcvb]

FYI, the idea for this enhancement originates from framework issue #2122.