minor #14825 Documenting how to keep option value BC - see #14377 (weaverryan)

This PR was merged into the 2.7 branch.

Discussion
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Documenting how to keep option value BC - see #14377

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #14377 (kinda)
| License       | MIT
| Doc PR        | symfony/symfony-docs#5179

Hi guys!

I'm still making sense of the form changes, but it seems that this before and after isn't totally honest - your option values will change, unless your add this extra option.

@webmozart look correct to you?

Thanks!

Commits
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deb9db8 Documenting how to keep option value BC - see #14377

UPGRADE-2.7.md

@@ -134,6 +134,11 @@ Form
            'Ignored' => Status::IGNORED,
        ),
        'choices_as_values' => true,
+       // important if you rely on your option value attribute (e.g. for JavaScript)
+       // this will keep the same functionality as before
+       'choice_value' => function ($choice) {
+           return $choice;
+       },
    ));
    ```