[Doc]Add documentation to benchmarking script when running TGI (#4920)

benchmarks/benchmark_serving.py

@@ -17,6 +17,10 @@ On the client side, run:
         --dataset-path <path to dataset> \
         --request-rate <request_rate> \ # By default <request_rate> is inf
         --num-prompts <num_prompts> # By default <num_prompts> is 1000
+        
+    when using tgi backend, add
+        --endpoint /generate_stream
+    to the end of the command above.
 """
 import argparse
 import asyncio