minor symfony#35297 Improve upgrading instructions for deprecated router options (flack)

This PR was merged into the 4.4 branch.

Discussion
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Improve upgrading instructions for deprecated router options

See symfony#30249 (comment)

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | symfony#30249
| License       | MIT
| Doc PR        |

Commits
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2cc7083 Improve upgrading instructions for deprecated router options

Author: fabpot

UPGRADE-5.0.md

@@ -393,7 +393,7 @@ Routing
 -------
 
  * The `generator_base_class`, `generator_cache_class`, `matcher_base_class`, and `matcher_cache_class` router
-   options have been removed.
+   options have been removed. If you are using multiple Router instances and need separate caches for them, set a unique `cache_dir` per Router instance instead.
  * `Serializable` implementing methods for `Route` and `CompiledRoute` are final.
    Instead of overwriting them, use `__serialize` and `__unserialize` as extension points which are forward compatible
    with the new serialization methods in PHP 7.4.