Documentation follow-up fixes for hook_update_last_removed() from #3098475

Problem/Motivation

Follow-up for #3098475: Add more strict checking of hook_update_last_removed() and better explanation

1. +++ b/core/modules/system/tests/modules/update_test_last_removed/update_test_last_removed.install
   @@ -0,0 +1,20 @@
   + * Update functions for the update_test_invalid_hook module.
   

   Whoops. This is wrong here, and in the patch already committed to 9.0.x and 8.9.x.

2. +++ b/core/modules/system/tests/src/Functional/UpdateSystem/UpdatePathLastRemovedTest.php
   @@ -0,0 +1,72 @@
   + * Modules can define their last removed update function.
   

   True statement. ;) Is that what we want this class comment to say?

3. setUp() is missing the {@inheritdoc}

Proposed resolution

Fix those comments.

Remaining tasks

1. Do eeet. See #2
2. Reviews/refinements.
3. RTBC.
4. Commit to 9.1.x, cherry-pick to 9.0.x and 8.9.x branches.
5. Wait for #3098475: Add more strict checking of hook_update_last_removed() and better explanation backport to land in 8.8.x branch.
6. Cherry-pick fix to 8.8.x branch.

User interface changes

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A