add comments to the tests

Author: dconeybe

packages/firestore/test/integration/api/transactions.test.ts

@@ -316,6 +316,11 @@ apiDescribe('Database transactions', (persistence: boolean) => {
     });
   });
 
+  // This test is identical to the test above, except that withNonexistentDoc()
+  // is replaced by withDeletedDoc(), to guard against regression of
+  // https://github.com/firebase/firebase-js-sdk/issues/5871, where transactions
+  // would incorrectly fail with FAILED_PRECONDITION when operations were
+  // performed on a deleted document (rather than a non-existent document).
   it('runs transactions after getting a deleted document', async () => {
     return withTestDb(persistence, async db => {
       const tt = new TransactionTester(db);

packages/firestore/test/lite/integration.test.ts

@@ -524,6 +524,11 @@ describe('Transaction', () => {
     });
   });
 
+  // This test is identical to the test above, except that a non-existent
+  // document is replaced by a deleted document, to guard against regression of
+  // https://github.com/firebase/firebase-js-sdk/issues/5871, where transactions
+  // would incorrectly fail with FAILED_PRECONDITION when operations were
+  // performed on a deleted document (rather than a non-existent document).
   it('can read deleted doc then write', () => {
     return withTestDocAndInitialData({ counter: 1 }, async doc => {
       await deleteDoc(doc);