docs: Fix KV cache transfer UCX configuration instructions

[nnshah1]

Summary

• Fix incorrect documentation stating that TRTLLM_USE_UCX_KV_CACHE=1 with backend: DEFAULT enables UCX
• This combination does not work - users must explicitly set backend: UCX
• Added clarifying note to prevent user confusion

Reference

• Linear: DYN-1678

Test plan

• Verify documentation renders correctly

🤖 Generated with Claude Code

Summary by CodeRabbit

• Documentation
  • Clarified KV cache transfer enablement requirements: explicitly requires setting backend: UCX in the engine configuration YAML.
  • Removed guidance about alternative configuration methods.
  • Added prominent notes emphasizing that environment variables alone do not enable UCX and explicit backend configuration is mandatory.

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[coderabbitai]

Walkthrough

Documentation update to KV cache transfer configuration guide clarifying the requirement to explicitly set cache_transceiver_config.backend: UCX in engine configuration YAML, removing alternative guidance and adding caution notes about common misconfigurations.

Changes

Cohort / File(s)	Summary
KV Cache Transfer Documentation docs/backends/trtllm/kv-cache-transfer.md	Reworded UCX enablement instructions to explicitly require setting backend: UCX in configuration YAML. Removed alternative method guidance using environment variable TRTLLM_USE_UCX_KV_CACHE with DEFAULT backend. Added caution note clarifying that environment variable with DEFAULT does not enable UCX and backend: UCX must be explicitly set.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Poem

🐰 A hop through the docs, oh what a delight,
UCX configuration shining clear and bright,
No more confusion, the path is now true—
Set backend: UCX, that's what you must do!
With caution notes dancing where doubts used to play. ✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 inconclusive)

Check name	Status	Explanation	Resolution
Description check	❓ Inconclusive	The description covers the main issue and includes a reference, but lacks details on specific file changes, reviewer guidance, and doesn't follow the provided template structure with Overview, Details, and Where should the reviewer start sections.	Restructure the description to follow the template with Overview, Details, and Where should the reviewer start sections. Add specific file paths and lines changed for reviewer clarity.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title clearly and specifically describes the main change: fixing documentation about KV cache transfer UCX configuration, which aligns with correcting misleading configuration instructions.

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