docs: document tool execution concurrency (#3164)

Author: seratch

docs/running_agents.md

@@ -152,14 +152,37 @@ Use `RunConfig` to override behavior for a single run without changing each agen
 -   [`workflow_name`][agents.run.RunConfig.workflow_name], [`trace_id`][agents.run.RunConfig.trace_id], [`group_id`][agents.run.RunConfig.group_id]: Sets the tracing workflow name, trace ID and trace group ID for the run. We recommend at least setting `workflow_name`. The group ID is an optional field that lets you link traces across multiple runs.
 -   [`trace_metadata`][agents.run.RunConfig.trace_metadata]: Metadata to include on all traces.
 
-##### Tool approval and tool error behavior
+##### Tool execution, approval, and tool error behavior
 
+-   [`tool_execution`][agents.run.RunConfig.tool_execution]: Configure SDK-side execution behavior for local tool calls, such as limiting how many function tools run at once.
 -   [`tool_error_formatter`][agents.run.RunConfig.tool_error_formatter]: Customize the model-visible message when a tool call is rejected during approval flows.
 
 Nested handoffs are available as an opt-in beta. Enable the collapsed-transcript behavior by passing `RunConfig(nest_handoff_history=True)` or set `handoff(..., nest_handoff_history=True)` to turn it on for a specific handoff. If you prefer to keep the raw transcript (the default), leave the flag unset or provide a `handoff_input_filter` (or `handoff_history_mapper`) that forwards the conversation exactly as you need. To change the wrapper text used in the generated summary without writing a custom mapper, call [`set_conversation_history_wrappers`][agents.handoffs.set_conversation_history_wrappers] (and [`reset_conversation_history_wrappers`][agents.handoffs.reset_conversation_history_wrappers] to restore the defaults).
 
 #### Run config details
 
+##### `tool_execution`
+
+Use `tool_execution` when you want the SDK to limit local function-tool concurrency for a run.
+
+```python
+from agents import Agent, RunConfig, Runner, ToolExecutionConfig
+
+agent = Agent(name="Assistant", tools=[...])
+
+result = await Runner.run(
+    agent,
+    "Run the required tool calls.",
+    run_config=RunConfig(
+        tool_execution=ToolExecutionConfig(max_function_tool_concurrency=2),
+    ),
+)
+```
+
+`max_function_tool_concurrency=None` preserves the default behavior: when a model emits multiple function tool calls in a turn, the SDK starts all emitted local function tool calls. Set an integer value to cap how many of those local function tools run at once.
+
+This is separate from provider-side [`ModelSettings.parallel_tool_calls`][agents.model_settings.ModelSettings.parallel_tool_calls]. `parallel_tool_calls` controls whether the model is allowed to emit multiple tool calls in a single response. `tool_execution.max_function_tool_concurrency` controls how the SDK executes local function tool calls after the model has emitted them.
+
 ##### `tool_error_formatter`
 
 Use `tool_error_formatter` to customize the message that is returned to the model when a tool call is rejected in an approval flow.