Correct usage_limits=None docstring on AgentOperator and LLMOperator

[kaxil]

The usage_limits docstrings on AgentOperator and LLMOperator say None (the default) "means no enforcement". That is wrong: pydantic-ai replaces None with UsageLimits() (usage_limits = usage_limits or UsageLimits() in Agent.iter), so its default request_limit=50 still applies and a long tool loop fails with UsageLimitExceeded at 50 requests. That is the failure the run captured in #74297 hit with usage_limits unset.

The docstrings now say None sets no token, cost, or tool-call limits but keeps the 50-request cap. The LLMOperator docstring also drops the sentence implying that moving from None to a cost_limit-only dict newly introduces the cap. Both already have it. The LLM* operators that inherit usage_limits from LLMOperator pick up the fix, and the how-to pages already describe the default correctly.

Checked against pydantic-ai-slim 2.33.0 (the provider's floor) and 2.48.0: a FunctionModel that always calls a tool raises UsageLimitExceeded: The next request would exceed the request_limit of 50 with usage_limits=None, and UsageLimits(request_limit=None) runs past 50 requests.

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