[terminal] IJ-MR-158015 Clarify EventFlowState thread usage

The initial wording was wrong: it does get called from
the terminal emulator thread sometimes,
on history overflow, when it's not the scrapping coroutine,
but the text buffer itself that initiates buffer collection.

GitOrigin-RevId: 75b99b8040bce96ce6bcb7be04d152ae1a286f80

plugins/terminal/backend/src/com/intellij/terminal/backend/fus/BackendLatencyServiceImpl.kt

@@ -196,7 +196,7 @@ private class BackendOutputActivityImpl : BackendOutputActivity {
   /**
    *  The part of the state that is affected by collecting the text buffer and further event processing.
    *
-   *  Accessed from different threads, though never from the terminal emulator thread.
+   *  Accessed from different threads, so must be thread-safe.
    */
   private class EventFlowState {
     private val collectedRanges = ConcurrentHashMap<IdentityWrapper<TerminalContentUpdatedEvent>, LongRange>()