#1539 docs: --pipeline-windows and --adapt-async 1 combine, not exclude (#1447)

描述

## Summary

- `docs/MULTI_GPU.md` claimed `--pipeline-windows` and `--adapt-async 1` exclude each other. In `src/program/generate.cpp` the asynchronous adaptive tier is only turned off beside `--pipeline-windows 2` when `STRATA_PIPELINE_ADAPT_ASYNC=0` is set; by default they run together, with swap steps waiting for the windows in flight that may still read what they overwrite (matching the reporter's startup-log observation and measurement table).
- Reworded that paragraph to describe the combination and the opt-out variable.

Fixes the doc-mismatch half of #1447 (the `--remote-expert-opt` remark there is a question about setup behavior, not this doc line — untouched).

## Test plan

- [x] Verified against `adapt_async_off` gating logic in `src/program/generate.cpp` (~line 5965)
- [ ] Docs-only change; no code paths affected