Small docs improvements of WebTestBase::rebuildContainer

Hm. The new docs look a bit confusing. If the kernel is not updated by this method, then I think we should just take out the mention of the kernel from the docs? As it is I was confused about the kernel mention.

I'm also not sure that the $request = \Drupal::request(); is a complete no-op. It doesn't seem to break anything in the automated tests, but the method is capable of actually resetting an array pointer if you trace it down the stack. And in any case, if you're going to remove that line of code this needs to move to the simpletest component as it's not just docs.

Looks good to me.
The part with the kernel instance is not necessary.

Test code is not subject to beta restrictions. Committed e40099f and pushed to 8.0.x. Thanks!

diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 9808cd0..51a46cc 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -1138,13 +1138,10 @@ protected function writeCustomTranslations() {
   /**
    * Rebuilds \Drupal::getContainer().
    *
-   * Use this to build a new service container. For example, when the
-   * list of enabled modules is changed via the internal browser, in which case
-   * the test process still contains the kernel and the old service container
-   * with an old module list.
-   *
-   * Note: This rebuild process is handled by the kernel, so you don't get a new
-   *   instance of it.
+   * Use this to update the test process's kernel with a new service container.
+   * For example, when the list of enabled modules is changed via the internal
+   * browser the test process's kernel has a service container with an out of
+   * date module list.
    *
    * @see TestBase::prepareEnvironment()
    * @see TestBase::restoreEnvironment()

I improved the comment. The unused $request has been bothering me for ages.