Document new process instance migration restrictions for boundary events

[korthout]

With 8.6, it's possible to map boundary events (and thus migrate their respective event subscriptions). While this adds more possibilities, we added several new restrictions to these new options. They should of course be documented in the API and the concept limitations:

• Ensure instance migration can't detach boundary event from element instance camunda#19956
• Ensure instance migration can't merge two boundary event subscriptions camunda#19960

For the API, it's easiest to just align it directly with the gateway.proto file.

For the limitations, we should describe these cases in the first section (limits that won't be lifted in the future).

[korthout]

Completed with:

• Document instance migration phases 3 and 4 #4348