Documentation update: use concurrency terminology in benchmark docstrings (facebook#4924)

Summary:

Updates docstrings in `BenchmarkMethod`, `BenchmarkExecutionSettings`, and `nightly.py` to use "concurrency" terminology instead of "parallelism" where appropriate. Also applies a formatting fix to a multi-line `BenchmarkExecutionSettings(...)` call in `nightly.py`. No interface or behavioral changes.

Differential Revision: D93771883

Author: mgrange1998

ax/benchmark/benchmark_method.py

@@ -16,7 +16,7 @@
 class BenchmarkMethod(Base):
     """Benchmark method, represented in terms of Ax generation strategy (which tells us
     which models to use when) and Orchestrator options (which tell us extra execution
-    information like maximum parallelism, early stopping configuration, etc.).
+    information like maximum concurrency, early stopping configuration, etc.).
 
     Args:
         name: String description. Defaults to the name of the generation strategy.