docs: explain budget denials during LLM retries and fallback

[ak684]

HUMAN:

• A human has tested these changes.

AGENT:

Summary of changes

Explain that LiteLLM budget denials stop retries and fallback, including denials from a fallback model. Temporary rate limits retain retries; raising/resetting the budget permits a later attempt.

Companion to the OHE-3324 fix in OpenHands/software-agent-sdk#5309. Validation: prose-only changes; git diff --check passes.

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

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