Added upgrade info for #9601
minor #9972 Upgrade File for 2.5 (Danez)

This PR was merged into the 2.5-dev branch.

Discussion
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Upgrade File for 2.5

Added upgrade info for #9601, as this pr might break code which overwrites this method and also to respect symfony/symfony-docs#3439.

Commits
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fefcf41 Added upgrade info for #9601