docs: clarify strict_mcp_config docstring

--strict-mcp-config does not control error handling for invalid MCP
configs; it restricts the CLI to only use MCP servers passed via
mcp_servers/--mcp-config, ignoring filesystem and settings-based MCP
configurations (.mcp.json, user/global settings, plugins). Update the
docstring to describe the actual behavior.

Author: qing-ant

src/claude_agent_sdk/types.py

@@ -1509,8 +1509,11 @@ class ClaudeAgentOptions:
     """
 
     strict_mcp_config: bool = False
-    """When ``True``, the CLI raises an error on invalid MCP server configuration
-    instead of warning to stderr. Matches the TypeScript SDK's ``strictMcpConfig``."""
+    """When ``True``, only use MCP servers passed via :attr:`mcp_servers`,
+    ignoring all other MCP configurations the CLI would otherwise load (e.g.
+    project ``.mcp.json``, user/global settings, plugin-provided servers).
+    Maps to the CLI's ``--strict-mcp-config`` flag and matches the TypeScript
+    SDK's ``strictMcpConfig`` option."""
 
     permission_mode: PermissionMode | None = None
     """Permission mode for the session.