Added documentation on setting process variables for method messageEventReceived

Discussed with Tijs. The current documentation on how variables are set is not very explicit on the underlying implementation and thus the consequences.

I have applied the documentation to RuntimeService.messageEventReceived, but I think there should be references from other places and the actual documentation should become part of VariableScopeImpl or something where this logic is actually implemented. Please suggest so I can adjust if needed.