Document SoundCloud async JS limitation

[kocaemre]

Summary

• Remove the incorrect "same args/returns" wording from SoundCloudScraper.scrape_async.
• Document that the async SoundCloud path is HTTP-only and does not support render_js/scroll_pages.
• Add a regression guard for the async docstring so the limitation stays explicit.

Verification

• RED: python -m pytest tests/test_scrapers/test_music.py::TestSoundCloudScraper::test_async_docstring_documents_http_only_limitation -q failed because the docstring still claimed parity.
• GREEN: python -m pytest tests/test_scrapers/test_music.py::TestSoundCloudScraper -q
• python -m pytest tests/ -q
• ruff check src/ tests/

Fixes #130.

[vedaant00]

Thanks @kocaemre, merged. This is exactly the resolution #130 called for: the docstring no longer claims parity it does not have, it names render_js/scroll_pages as sync-only, and points async users at the right method. Nice touch adding a regression test that asserts both the honest wording is present and the old "same args/returns" phrasing is gone, so it cannot silently creep back. Appreciate the clean contribution.