docs(libmdbx): add missing # Panics documentation for unwrap calls

[letmehateu]

Problem

Several functions in the transaction module use .unwrap() on channel receives and type
conversions without documenting the panic conditions.

Summary

Adds missing # Panics documentation to 4 public functions and 1 Drop implementation
in libmdbx-rs/transaction.rs that call .unwrap() but don't document when they panic.