add comment about description

Author: AbhiPrasad

packages/core/src/tracing/transaction.ts

@@ -44,6 +44,8 @@ export class Transaction extends SpanClass implements TransactionInterface {
    */
   public constructor(transactionContext: TransactionContext, hub?: Hub) {
     super(transactionContext);
+    // We need to delete description since it's set by the Span class constructor
+    // but not needed for transactions.
     delete this.description;
 
     this._measurements = {};