docs(spanner): update isolation level terminology in comments (#15900)

Author: prathmeshcc

google/cloud/spanner/transaction.h

@@ -83,7 +83,7 @@ class Transaction {
      * absence of conflicts between its updates and any concurrent updates
      * that have occurred since that snapshot. Consequently, in contrast to
      * `kSerializable` transactions, only write-write conflicts are detected in
-     * snapshot transactions.
+     * repeatable read transactions.
      */
     kRepeatableRead,
   };

google/cloud/spanner/transaction_test.cc

@@ -188,7 +188,7 @@ TEST(Transaction, IsolationLevelPrecedence) {
         return 0;
       });
 
-  // Case 2: Fallback to client default
+  // Case 2: Fallback to default options
   auto opts_default = Transaction::ReadWriteOptions();
   Transaction txn_default = MakeReadWriteTransaction(opts_default);
   spanner_internal::Visit(
@@ -202,7 +202,7 @@ TEST(Transaction, IsolationLevelPrecedence) {
 }
 
 TEST(Transaction, IsolationLevelNotSpecified) {
-  // Case: Isolation not specified in transaction level or client level
+  // Case: Isolation not specified in transaction options or default options
   auto opts = Transaction::ReadWriteOptions();
   Transaction txn = MakeReadWriteTransaction(opts);
   spanner_internal::Visit(