[Bank Account]: Added Explicit Instruction Append for not Doing Concurrency Tests. (#3864)

* Added explicit instruction append for not doing concurrency tests.

* Apply suggestions from code review

Co-authored-by: Isaac Good <[email protected]>

---------

Co-authored-by: Isaac Good <[email protected]>

Author: BethanyG

exercises/practice/bank-account/.docs/instructions.append.md

@@ -1,5 +1,22 @@
 # Instructions append
 
+````exercim/note
+Python doesn't support "true" concurrency due to the [Global Interpreter Lock][GIL].
+While work is ongoing to create support for [free-threading in Python][free-threading], it is still experimental.
+Current standard library solutions such as [multiprocessing][multiprocessing-module] and [threading][threading-module] are difficult to implement with the current track tooling.  
+
+
+As a result, the concurrency requirement has been set aside for this exercise. 
+Account operations are sequential on a single thread, and no concurrency or "race condition" tests are run.
+
+[GIL]: https://realpython.com/python-gil/
+[free-threading]: https://docs.python.org/3/howto/free-threading-python.html
+[threading-module]: https://docs.python.org/3/library/threading.html#module-threading
+[multiprocessing-module]: https://docs.python.org/3/library/multiprocessing.html#sharing-state-between-processes
+````
+
+<br>
+
 ## Exception messages
 
 Sometimes it is necessary to [raise an exception](https://docs.python.org/3/tutorial/errors.html#raising-exceptions). When you do this, you should always include a **meaningful error message** to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. For situations where you know that the error source will be a certain type, you can choose to raise one of the [built in error types](https://docs.python.org/3/library/exceptions.html#base-classes), but should still include a meaningful message.