Document use of unsafe in builder.rs

[alamb]

Note: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-10977

There are ~6 uses of unsafe in builder.rs 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}