I can implement a custom history level

AT:

• I can implement a custom history level by restricting the set of HistoryEvents produced by the process engine
• I can fine-tune logging of history data for individual process definitions, individual activities, individual variables etc.
• I can use the name of my custom history level in the process engine configuration to make the customization explicit

docs should be enhanced by a section "Implementing a Custom History Level" including an example.