#1477 docs: correct pipeline async compatibility (#1447)

Description

Fixes #1447.

The pipeline section of `docs/MULTI_GPU.md` says that `--pipeline-windows` and `--adapt-async 1` exclude each other, although the same section and `docs/DETAILS.md` describe their combined operation.

Correct the stale paragraph: `--pipeline-windows 2` can run with `--adapt-async 1`. `STRATA_PIPELINE_ADAPT_ASYNC=0` keeps the blocking adaptive tier with the pipeline. Clarify that this switch does not require `STRATA_PIPELINE_DEBUG=1`.

Validation: checked the `pl_async` condition and fallback in `src/program/generate.cpp:5965-5979` against `docs/DETAILS.md:169-174`; `git diff --check` passes. One documentation paragraph changed. No GPU run.