docs: add cloud storage access documentation (#567)

## Summary

Document the `fsspec`-based cloud storage access feature (added in PR
#523) which was never documented.

## Content

New "Cloud Storage Access" section in `docs/io.rst` covering:
- Supported protocols: `s3://`, `gs://`, `az://`, `azureml://`
- Backend package installation (`s3fs`, `gcsfs`, `adlfs`)
- Usage examples with `rdrecord` and `rdann`
- Authentication guidance
- Note clarifying that cloud URIs go in `record_name`, not `pn_dir`

Closes #547

Author: briangow

docs/io.rst

@@ -30,6 +30,58 @@ WFDB Annotations
     :members: wrann
 
 
+Cloud Storage Access
+--------------------
+
+WFDB-Python supports reading records and annotations directly from cloud
+storage via the ``fsspec`` library. Pass a cloud URI as the
+``record_name`` argument instead of a local path.
+
+**Supported protocols:** ``s3://`` (Amazon S3), ``gs://`` (Google Cloud
+Storage), ``az://`` (Azure Blob Storage), and ``azureml://`` (Azure ML).
+
+**Prerequisites:** Install the ``fsspec`` backend for your cloud provider:
+
+.. code-block:: bash
+
+    pip install s3fs      # Amazon S3
+    pip install gcsfs     # Google Cloud Storage
+    pip install adlfs     # Azure Blob Storage
+
+``fsspec`` itself is already included as a core dependency of ``wfdb``.
+
+**Usage examples:**
+
+.. code-block:: python
+
+    import wfdb
+
+    # Read a record from Amazon S3
+    record = wfdb.rdrecord("s3://my-bucket/wfdb-data/100")
+
+    # Read from Google Cloud Storage
+    record = wfdb.rdrecord("gs://my-bucket/wfdb-data/100")
+
+    # Read annotations from S3
+    ann = wfdb.rdann("s3://my-bucket/wfdb-data/100", "atr")
+
+    # For PhysioNet databases, use pn_dir with the database name:
+    record = wfdb.rdrecord("100", pn_dir="mitdb")
+    ann = wfdb.rdann("100", "atr", pn_dir="mitdb")
+
+**Authentication:** Configure credentials through the standard
+provider-specific mechanism (e.g., ``~/.aws/credentials`` for S3,
+``GOOGLE_APPLICATION_CREDENTIALS`` for GCS). The ``fsspec`` library
+handles credential discovery automatically.
+
+.. note::
+
+    Cloud URIs must be passed as ``record_name``, not ``pn_dir``.
+    The ``pn_dir`` parameter is reserved for PhysioNet database names
+    (e.g., ``"mitdb"`` or ``"mimic4wdb/0.1.0"``), which are resolved
+    against the configured PhysioNet index URL.
+
+
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