Add comment on the blocking behavior of Run

Signed-off-by: Matej Pavlovic <matopavlovic@gmail.com>

docs/architecture-decision-records/0003-generalize-modules.md

@@ -68,6 +68,10 @@ This approach:
         // Note, however, that the implementation may produce output events even without receiving any input.
         // Before Run is called, ActiveModule guarantees not to read or write,
         // respectively, from eventsIn and to eventsOut.
+        //
+        // In general, Run is expected to block until explicitly asked to stop
+	    // (through a module-specific event or by canceling ctx)
+        // and thus should most likely be run in its own goroutine.
         // Run must return after ctx is canceled.
         //
         // Note that the node does not guarantee to always read events from eventsOut.