[OptionsResolver] Documented BC break
minor #12384 [OptionsResolver] Documented BC break (webmozart)

This PR was merged into the 2.6-dev branch.

Discussion
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[OptionsResolver] Documented BC break

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12378
| License       | MIT
| Doc PR        | -

Commits
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0b0bfc0 [OptionsResolver] Documented BC break