D12 compat: add missing description_classes on before branch (CR#3482370)

Andrii Podanenkorequested to merge
d12-form-element-description-class into 4.x

Mirrors core's own change-record #3482370 (issue #2700439): form-element.html.twig only applied .addClass(description_classes) on the after/invisible description-display branch; the before branch was missing the same classes. Pure CSS-class parity fix, no functional change.

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