Missing safety comments for calling ArrowDataBuilder::build_unchecked in Parquet

[jp0317]

Describe the bug
There are some places in the Parquet codes (mostly in arrow interfaces, e.g., here and here) calling the ArrowDataBuilder::build_unchecked without clear safety comments. In particular, it is not straightforwardly clear that whether the inputs do form valid Arrow Array.

To Reproduce
n/a

Expected behavior
Calling unsafe functions should have clear safety comments

Additional context

[alamb]

Thank you @jp0317