Document use of unsafe in datatypes.rs

[alamb]

Note: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-10983

There are ~12 uses of unsafe in datatypes and we should document them according to the guidelines in the Arrow crate README

 
{code:java}
// JUSTIFICATION
// Benefit
// Describe the benefit of using unsafe. E.g.
// "30% performance degradation if the safe counterpart is used, see bench X."
// Soundness
// Describe why the code remains sound (according to the definition of rust's unsafe code guidelines). E.g.
// "We bounded check these values at initialization and the array is immutable."
let ... = unsafe { ... }; {code}

[ByteBaker]

No file with the name exists. I could find datatype.rs in arrow-schema which had no instances of unsafe. I found data_type.rs in parquet with 10 instances of unsafe. Not sure if the issue represents which one, since the context is old now.

Your call @alamb

[alamb]

I agree this isn't tracking anything actionable now. Let's close this and file specific tickets when we encounter undocumented unsafe code