[No QA] Add a comment explaining handleUnusedOptimisticID

src/libs/Middleware/HandleUnusedOptimisticID.ts

@@ -5,6 +5,16 @@ import * as PersistedRequests from '@userActions/PersistedRequests';
 import ONYXKEYS from '@src/ONYXKEYS';
 import type Report from '@src/types/onyx/Report';
 
+/**
+ * This middleware checks for the presence of a field called preexistingReportID in the response.
+ * If present, that means that the client passed an optimistic reportID with the request that the server did not use.
+ * This can happen because there was already a report matching the parameters provided that the client didn't know about.
+ * (i.e: a DM chat report with the same set of participants)
+ *
+ * If that happens, this middleware checks for any serialized network requests that reference the unused optimistic ID.
+ * If it finds any, it replaces the unused optimistic ID with the "real ID" from the server.
+ * That way these serialized requests function as expected rather than returning a 404.
+ */
 const handleUnusedOptimisticID: Middleware = (requestResponse, request, isFromSequentialQueue) =>
     requestResponse.then((response) => {
         const responseOnyxData = response?.onyxData ?? [];