Missing safety comments for calling ArrowDataBuilder::build_unchecked in Parquet

**Describe the bug**
There are some places in the Parquet codes (mostly in arrow interfaces, e.g., [here](https://github.com/apache/arrow-rs/blob/main/parquet/src/arrow/array_reader/list_array.rs#L227) and [here](https://github.com/apache/arrow-rs/blob/main/parquet/src/arrow/array_reader/primitive_array.rs#L182)) calling the [ArrowDataBuilder::build_unchecked](https://github.com/apache/arrow-rs/blob/main/arrow-data/src/data.rs#L1902-L1906) 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**

Start with the call sites in parquet/src/arrow/array_reader/list_array.rs and primitive_array.rs, then read ArrowDataBuilder::build_unchecked in arrow-data/src/data.rs to understand its safety requirements. Search the Parquet code for other calls and add clear comments explaining why each input forms a valid Arrow array; the work is done when all relevant calls document their safety assumptions.

Written by the indexing model from the issue text.