minor #32915 [Routing] Add a warning about the getRouteCollection() method (fabpot)

This PR was merged into the 3.4 branch.

Discussion
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[Routing] Add a warning about the getRouteCollection() method

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

In #19274, we tried to deprecate `RouterInterface::getRouteCollection()`, but failed at doing so. I propose to add a warning about why one should never use it at runtime as a first step.

Commits
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8863f0675d [Routing] added a warning about the getRouteCollection() method

Author: fabpot

RouterInterface.php

@@ -26,6 +26,9 @@ interface RouterInterface extends UrlMatcherInterface, UrlGeneratorInterface
     /**
      * Gets the RouteCollection instance associated with this Router.
      *
+     * WARNING: This method should never be used at runtime as it is SLOW.
+     *          You might use it in a cache warmer though.
+     *
      * @return RouteCollection A RouteCollection instance
      */
     public function getRouteCollection();