[Form] fix iterator typehint
merged branch Tobion/form-phpdoc (PR #8925)

This PR was merged into the 2.3 branch.

Discussion
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[Form] fix iterator phpdoc

It returns IteratorAggregate. But we probably only want to specify Traversable because that is was counts.

Fix from #8852

Commits
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f438ac3 [Form] fix iterator typehint