minor #29371 Update HttpKernel.php (shreypuranik)

This PR was merged into the 3.4 branch.

Discussion
----------

Update HttpKernel.php

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      |  no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

This update adds a docblock to varToString method in HttpKernel Class

Commits
-------

3324e2a2a6 Update HttpKernel.php

src/Symfony/Component/HttpKernel/HttpKernel.php

@@ -263,6 +263,9 @@ class HttpKernel implements HttpKernelInterface, TerminableInterface
         }
     }
 
+    /**
+     * Returns a human-readable string for the specified variable.
+     */
     private function varToString($var)
     {
         if (\is_object($var)) {