[Doc] Sync KV event medium terminology after #48123

[Alex-ai-future]

Summary

The KV event medium wire contract for filesystem and object-store secondary tiers was collapsed from per-tier FS / OBJ to a single coarse-grained STORAGE value in #48123 (commit 77cba0259f, merged 2026-07-27). The user-facing docs/features/kv_offloading_usage.md still described FS and OBJ as the medium emitted by each tier, which would mislead any KV event subscriber implementing against the doc.

This PR updates the doc to:

• State that both filesystem and object-store tiers emit medium STORAGE.
• Point readers at the optional tier locality field (introduced in #48281) for location semantics, instead of the medium.
• Drop the ambiguous "OBJ does not imply REMOTE" sub-clause on the object-store locality row, which mixed tier-type terminology with event-medium terminology.
• Leave the Filesystem (FS) / Object Store (OBJ) section headings, the type: "fs" / type: "obj" config values, and the NIXL OBJ backend name untouched — those are tier-type or NIXL-backend identifiers, not event-medium values.

Why this is not duplicating an existing PR

• A search of open PRs/issues for kv_offloading_usage, MEDIUM_STORAGE, kv event medium, and locality turned up no other doc fix targeting this drift. The only open PR touching this same file is #49048 ("Document SimpleCPUOffloadConnector"), which only adds a new top/bottom section (line 5, line 207+); its diff region is disjoint from this PR's edits at lines 116/130/171.
• RFC #49413 ("KV offload event path refactor") explicitly lists Medium/locality as out of scope and confirms the STORAGE / locality values will not change in the upcoming event path refactor.

Changes

docs/features/kv_offloading_usage.md only, 5 insertions / 5 deletions across three locations:

1. Secondary Tiers overview paragraph — explain that both tiers use medium STORAGE, and redirect readers to locality for location semantics.
2. Filesystem (type: "fs") enable_kv_events row — medium \FS`→medium `STORAGE``.
3. Object-store (type: "obj") enable_kv_events row — medium \OBJ`→medium `STORAGE`; same row's localityentry loses the; OBJ does not imply `REMOTE`` sub-clause (now redundant with the overview wording).

Test commands run

# Markdown lint on the changed file
pre-commit run markdownlint-cli2 --files docs/features/kv_offloading_usage.md
# -> Passed

# Typos check on the changed file
pre-commit run typos --files docs/features/kv_offloading_usage.md
# -> Passed

# Confirm no stale FS / OBJ medium tokens remain
grep -nE 'medium.*FS|medium.*OBJ|MEDIUM_FS|MEDIUM_OBJ' docs/features/kv_offloading_usage.md
# -> exit 1, no matches

# Confirm code-side wire contract still uses Medium.STORAGE
grep -n 'Medium\.STORAGE\|MEDIUM_STORAGE' vllm/distributed/kv_events.py \
  vllm/v1/kv_offload/tiering/fs/manager.py \
  vllm/v1/kv_offload/tiering/obj/manager.py
# -> 4 matches, all consistent with the doc

AI assistance was used

An AI assistant (Claude) drafted the doc edits, the commit message, and this PR description. All factual claims were verified by reading the code and the cited PRs/commits; no claim in this description or the commit was accepted without evidence.

[mergify]

Documentation preview: https://vllm--51646.org.readthedocs.build/en/51646/