Add comments on why WaitTerminateTestTrait is used

Problem/Motivation

In #3375959: Add a way to delay executions in test runner until terminate event completed in the child site we introduced the Drupal\Tests\WaitTerminateTestTrait, which made waiting on a lock in the existing test http client middleware possible.
This made it possible to remove the sleep(1) introduced in #3295790: Post-response task running (destructable services) are actually blocking; add test coverage and warn for common misconfiguration.
However, we also removed the comments on why those sleeps/locks are needed.

Since it's certainly not trivial reading the code why we are waiting, and a future refactor might negate the wait, let's re-introduce these comments.

Steps to reproduce

Proposed resolution

- Look at the comments removed in #3375959: Add a way to delay executions in test runner until terminate event completed in the child site (here's the commit: https://git.drupalcode.org/project/drupal/-/commit/1debb39) and reintroduce them in this issue.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet