docs: clarify LostInTheMiddleRanker threshold units

[zcxGGmu]

Related Issues

• Refs feat: support token-based budget in LostInTheMiddleRanker #11351

Proposed Changes:

• Clarify that LostInTheMiddleRanker.word_count_threshold counts whitespace-separated words, not tokenizer-aware tokens.
• Add the same clarification to the current and 2.31 versioned docs page.
• Keep runtime behavior unchanged, matching the maintainer guidance in feat: support token-based budget in LostInTheMiddleRanker #11351.

How did you test it?

• hatch run fmt-check haystack/components/rankers/lost_in_the_middle.py
• hatch -e test run pytest test/components/rankers/test_lost_in_the_middle.py
• git diff --check

Notes for the reviewer

No release note was added because this is a docs/docstring-only clarification with no behavior change.

This PR was generated with an AI assistant. I reviewed the changes and ran the relevant tests.

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights, or the PR body contains the relevant context.
• I have added unit tests and updated the docstrings, where applicable.
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I have documented my code.
• I have added a release note file, following the contributors guidelines, or the change is docs-only / not user-facing runtime behavior.
• I have run pre-commit hooks and fixed any issue, or run the focused equivalent checks listed above.

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