docs(NODE-6191): clarify that operations should not be parallelized in transactions (#4122)

Co-authored-by: Aditi Khare <[email protected]>

Author: W-A-James

src/sessions.ts

@@ -369,6 +369,11 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
   /**
    * Starts a new transaction with the given options.
    *
+   * @remarks
+   * **IMPORTANT**: Running operations in parallel is not supported during a transaction. The use of `Promise.all`,
+   * `Promise.allSettled`, `Promise.race`, etc to parallelize operations inside a transaction is
+   * undefined behaviour.
+   *
    * @param options - Options for the transaction
    */
   startTransaction(options?: TransactionOptions): void {
@@ -443,6 +448,11 @@ export class ClientSession extends TypedEventEmitter<ClientSessionEvents> {
    *
    * **IMPORTANT:** This method requires the function passed in to return a Promise. That promise must be made by `await`-ing all operations in such a way that rejections are propagated to the returned promise.
    *
+   * **IMPORTANT:** Running operations in parallel is not supported during a transaction. The use of `Promise.all`,
+   * `Promise.allSettled`, `Promise.race`, etc to parallelize operations inside a transaction is
+   * undefined behaviour.
+   *
+   *
    * @remarks
    * - If all operations successfully complete and the `commitTransaction` operation is successful, then the provided function will return the result of the provided function.
    * - If the transaction is unable to complete or an error is thrown from within the provided function, then the provided function will throw an error.