Document why BigPipe no-js placeholder rendering doesn't use Fibers

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IMHO we should not do this and instead just document why it's not worth it: #3383449-15: Add Fibers support to Drupal\Core\Render\Renderer → .

But … that assumes that 📌 Add Fibers support to Drupal\Core\Render\Renderer RTBC indeed lands of course, so let's wait for that to happen first?

Makes sense to me, downgrading to minor and changing the title. Leaving postponed on the two other issues.

They both landed.

Do you still agree with #2, @catch?

Yes I think we should just document why we haven't done it.