Dropdown.Button do not support overlayClassName

Editor assessment

The subject `fix: Dropdown.Button do not support overlayClassName` names the component and the exact unsupported prop, which is a complete defect statement. The fix type fits a missing pass-through, and the Dropdown.Button naming substitutes for a scope. No body is present, acceptable for a contained API fix whose subject says everything; no footer is required. Hygiene is good: grammatically blunt but clear, no junk, no trailing period. A reader knows precisely which capability was added and to what component.