minor #28808 [HttpKernel] Add a note about env name restrictions (fabpot)

This PR was merged into the 4.2-dev branch.

Discussion
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[HttpKernel] Add a note about env name restrictions

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | #26884
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

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Commits
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b0f863d77e [HttpKernel] added a note about env name restrictions

src/Symfony/Component/HttpKernel/Kernel.php

@@ -41,6 +41,9 @@ use Symfony\Component\HttpKernel\DependencyInjection\MergeExtensionConfiguration
  *
  * It manages an environment made of bundles.
  *
+ * Environment names must always start with a letter and
+ * they must only contain letters and numbers.
+ *
  * @author Fabien Potencier <fabien@symfony.com>
  */
 abstract class Kernel implements KernelInterface, RebootableInterface, TerminableInterface