add explanation of Multi-Turn Benchmark at the beginning of mt_bench.py

It is good practice to include docstrings with clarifications.

Signed-off-by: Costa Shulyupin <costa.shul@redhat.com>

Author: makelinux

src/instructlab/eval/mt_bench.py

@@ -1,5 +1,11 @@
 # SPDX-License-Identifier: Apache-2.0
 
+"""
+Multi-Turn Benchmark
+https://klu.ai/glossary/mt-bench-eval
+https://arxiv.org/html/2306.05685
+"""
+
 # First Party
 from instructlab.eval import (
     mt_bench_answers,