Document AsyncFS interface

[Eldergenix]

Change Description

Addresses #14021

This is a documentation-only first pass over the async filesystem API:

• documents the shared AsyncFS interface and lifecycle expectations
• documents RouterAsyncFS routing and backend ownership behavior
• documents the Google Cloud Storage, Amazon S3, and Azure Blob Storage async filesystem implementations

This intentionally does not rename methods, change public/private visibility, or convert positional arguments to keyword-only arguments. Those API-boundary changes need compatibility review and can follow separately.

Validation

• python3 -m py_compile hail/python/hailtop/aiotools/fs/fs.py hail/python/hailtop/aiotools/router_fs.py hail/python/hailtop/aiocloud/aiogoogle/client/storage_client.py hail/python/hailtop/aiocloud/aioaws/fs.py hail/python/hailtop/aiocloud/aioazure/fs.py
• git diff --check

Not run locally:

• make -C hail python-version-info / pytest collection - blocked because this environment has BSD make and no gmake; without generated version files pytest collection fails with ModuleNotFoundError: No module named 'hailtop.version'.
• make check-hail-fast - not run because local ruff/pyright tooling is not installed.
• docs build - not run; this change is inline docstrings only.

Security Assessment

• This change cannot impact the Hail Batch instance as deployed by Broad Institute in GCP

Impact Rating

• This change has no security impact

Impact Description

Documentation-only docstring updates for filesystem interfaces. No runtime behavior, public signatures, deployment code, authentication, authorization, networking, or Batch service paths are changed.