[docs] Document --scheduler-cls base class requirement (extend AsyncScheduler, not Scheduler)

[kliukovkin]

Purpose

Documents the requirement that custom scheduler plugins loaded via --scheduler-cls should extend AsyncScheduler, not Scheduler. The default V1 scheduler is AsyncScheduler; subclassing Scheduler instead disables async scheduling overlap with GPU execution and causes significant latency regression (we measured ~78% on multi-turn workloads before catching the mismatch).

Context

This footgun was identified during RFC #42185 (CacheAffinityScheduler) benchmark investigation. Took ~6 benchmark rounds to diagnose — the initial regression looked like a scheduling-logic bug but was entirely the sync-vs-async base class mismatch. Documenting it spares the next contributor that debugging cycle.

Changes

• vllm/config/scheduler.py — get_scheduler_cls() warning message now mentions the base class requirement and links to RFC [RFC]: Cache-affinity-aware request ordering for the V1 scheduler #42185 for context.
• vllm/v1/core/sched/scheduler.py — Scheduler class docstring notes that most custom plugins should use AsyncScheduler instead.
• vllm/v1/core/sched/async_scheduler.py — AsyncScheduler class docstring identifies it as the correct base class for plugins.

No behavior changes. Pure documentation + warning text improvement.

Test

pytest tests/v1/core/test_scheduler.py passes (96 tests, no behavior touched).

cc @joerunde (pluggable scheduler PR #14466 author) for awareness — this is a docs follow-up to the substrate that PR introduced.

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