Document why ViewsBlock::build() explicitly asks for an un-cached view.

Problem/Motivation

The \Drupal\views\Plugin\Block\ViewsBlock::build() function contains the following code

if ($output = $this->view->buildRenderable($this->displayID, [], FALSE)) {
  // ...
}

... the FALSE there means that the views output should not be render cached. But it's not clear why we do that; especially since if you look, the block is render-cached.

While trying to track down problems with the block cache, I talked to @dawehner (one of the primary authors of the patch which introduced this call, #2381277: Make Views use render caching and remove Views' own "output caching"), who explained that the real reason why we pass FALSE here is because we don't want to create a dedicated render cache for the view itself. Rather, we want to create a cache for the block only.

Proposed resolution

Since this is not clear, and could be a stumbling block / something that looks like a solution but is not for someone trying to debug render caching, I feel like we should add a short comment to explain why.

Remaining tasks

1. Write a patch
2. Review and feedback
3. RTBC
4. Commit

User interface changes

None.

API changes

None.

Data model changes

None.