Developer guide: handling of triggers

[cymed]

teksi/wastewater#381 introduces a new way of enabling/disabling triggers based on the name and the location of their corresponding trigger function:

• If a trigger is to be disabled when enabling/disabling symbology triggers, the function must be tww_app.symbology{__funcname}
• if a trigger automatically updates values that are part of the official model, the corresponding trigger function must be tww_app.autoupdate{__funcname} or tww_sys.update_last_modified{__funcname}