Document mHoldSync and mHoldReport (project-chip#13476)

src/app/ReadHandler.h

@@ -217,6 +217,10 @@ class ReadHandler : public Messaging::ExchangeDelegate
     uint16_t mMinIntervalFloorSeconds   = 0;
     uint16_t mMaxIntervalCeilingSeconds = 0;
     SessionHolder mSessionHandle;
+    // mHoldReport is used to prevent subscription data delivery while we are
+    // waiting for the min reporting interval to elapse.  If we have to send a
+    // report immediately due to an urgent event being queued,
+    // UnblockUrgentEventDelivery can be used to force mHoldReport to false.
     bool mHoldReport         = false;
     bool mDirty              = false;
     bool mActiveSubscription = false;
@@ -226,8 +230,12 @@ class ReadHandler : public Messaging::ExchangeDelegate
     NodeId mInitiatorNodeId                                  = kUndefinedNodeId;
     AttributePathExpandIterator mAttributePathExpandIterator = AttributePathExpandIterator(nullptr);
     bool mIsFabricFiltered                                   = false;
-    bool mHoldSync                                           = false;
-    uint32_t mLastWrittenEventsBytes                         = 0;
+    // mHoldSync is used to prevent subscription empty report delivery while we
+    // are waiting for the max reporting interval to elaps.  When mHoldSync
+    // becomes false, we are allowed to send an empty report to keep the
+    // subscription alive on the client.
+    bool mHoldSync                   = false;
+    uint32_t mLastWrittenEventsBytes = 0;
     SubjectDescriptor mSubjectDescriptor;
     // The detailed encoding state for a single attribute, used by list chunking feature.
     AttributeValueEncoder::AttributeEncodeState mAttributeEncoderState;