Spring Use Threadlocal.remove() instead of .set(null) in documentation

Hi Team,

This PR updates the Spring Framework documentation to recommend using ThreadLocal.remove() instead of ThreadLocal.set(null) when unsetting a ThreadLocal value.

The current documentation suggests using ThreadLocal.set(null) to unset a ThreadLocal value. However, this approach can lead to memory leaks and other unexpected behaviors, particularly in multi-threaded and multi-classloader environments. Using ThreadLocal.set(null) does not completely remove the ThreadLocal instance, which means the associated memory may not be fully released. This can cause data inconsistencies and unexpected behaviors.

In the Spring Framework itself, ThreadLocal.remove() is used to ensure that the ThreadLocal values are properly cleaned up, preventing potential memory leaks and ensuring consistent behavior across different environments.

Comment From: snicoll

Good catch, thanks @Seungpang