[sergiopaniego]

What does this PR do?

Follow-up to #7751, as suggested by @albertvillanova in the review: the vLLM TIP in grpo_trainer.md still says Truncated Importance Sampling is the default. Since #4530 the default vllm_importance_sampling_mode is sequence_mask, so the TIP now names it and links the Masked and Sequence-level IS sections of the paper index.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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AI writing disclosure

• No AI usage: the PR was written entirely by a human.
• AI-assisted: some parts were suggested or improved by AI, but the PR was written and reviewed by a human.
• AI-generated: the PR was mostly or fully generated by an AI tool.

[bot-ci-comment]

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