Document use of unsafe in parquet crate

*Note*: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-10984

There are ~64 uses of unsafe in the parquet crate
{code:java}
./parquet/src/util/hash_util.rs:6
./parquet/src/util/bit_packing.rs:34
./parquet/src/util/bit_util.rs:1
./parquet/src/data_type.rs:12
./parquet/src/arrow/record_reader.rs:5
./parquet/src/arrow/array_reader.rs:8 {code}