Fix description and example of on_event

[AArdeev]

Product: Tarantool
Root document: https://www.tarantool.io/en/doc/latest/platform/app/app_roles/#creating-a-custom-role
SME: @ totktonada, grafin

Details

Change desxription of the on_event function

• callback accepts 3 arguments, when it is called, rather than returns.
• "value, which shows and logs the information about the instance status as in the trigger box.status system event" - need to remove information about logging
• Example in the on_event section must be 1) changed to a more relevent one and 2) moved to the https://www.tarantool.io/en/doc/latest/platform/app/app_roles/#specifics-of-creating-spaces section
• A backwards link must be added from https://www.tarantool.io/en/doc/latest/platform/app/app_roles/#specifics-of-creating-spaces to the on_event section, explaining that space creation can now be defined in the on_event function

Follow up to #4666 and #5244