[rllib] Document auto-concat in a3c (#2533)

* docs

* update hyperparm docs

python/ray/rllib/agents/a3c/a3c.py

@@ -30,7 +30,8 @@ DEFAULT_CONFIG = with_common_config({
     "use_gpu_for_workers": False,
     # Whether to emit extra summary stats
     "summarize": False,
-    # Workers sample async
+    # Workers sample async. Note that this increases the effective
+    # sample_batch_size by up to 5x due to async buffering of batches.
     "sample_async": True,
     # Model and preprocessor options
     "model": {