doc: describe kSyncWriteAcceptedOnFalse stream sentinel

Author: trivikr

lib/internal/streams/iter/types.js

@@ -70,6 +70,12 @@ const kValidatedSource = Symbol('kValidatedSource');
  */
 const kSyncWriteAccepted = Symbol('kSyncWriteAccepted');
 
+/**
+ * Internal sentinel for writers whose sync write methods may return false
+ * after accepting data when backpressure is applied. Such writers must expose
+ * desiredSize so callers can distinguish accepted backpressure from a sync
+ * write that was not performed.
+ */
 const kSyncWriteAcceptedOnFalse = Symbol('kSyncWriteAcceptedOnFalse');
 
 module.exports = {