docs: add cloud URI access documentation

Document the fsspec-based cloud/remote file access feature that was
added in PR #523 but never documented. Covers:

- Supported protocols (S3, GCS, Azure, HTTPS)
- Installation of cloud extras
- Usage examples for rdrecord and rdann with remote paths
- Authentication guidance for credentialed cloud providers

Closes #547

Author: haoyu-haoyu

docs/io.rst

@@ -30,6 +30,49 @@ WFDB Annotations
     :members: wrann
 
 
+Cloud and Remote Access
+-----------------------
+
+WFDB-Python supports reading records and annotations directly from cloud
+storage and remote URLs via the ``fsspec`` library. Instead of downloading
+entire databases, you can access individual files on demand.
+
+**Supported protocols** include ``s3://`` (Amazon S3), ``gs://`` (Google
+Cloud Storage), ``az://`` (Azure Blob Storage), ``https://``, and any
+other protocol supported by ``fsspec``.
+
+**Installation:**
+
+.. code-block:: bash
+
+    pip install wfdb[cloud]
+    # or: pip install fsspec s3fs  (for S3 specifically)
+
+**Usage examples:**
+
+.. code-block:: python
+
+    import wfdb
+
+    # Read a record from an HTTPS URL
+    record = wfdb.rdrecord("100", pn_dir="https://physionet.org/files/mitdb/1.0.0/")
+
+    # Read from Amazon S3
+    record = wfdb.rdrecord("s3://my-bucket/wfdb-data/100")
+
+    # Read annotations from a remote path
+    ann = wfdb.rdann("100", "atr", pn_dir="https://physionet.org/files/mitdb/1.0.0/")
+
+**Authentication:** For cloud providers requiring credentials (S3, GCS,
+Azure), configure authentication through the standard provider-specific
+mechanism (e.g., ``~/.aws/credentials`` for S3, ``GOOGLE_APPLICATION_CREDENTIALS``
+for GCS). The ``fsspec`` library handles credential discovery automatically.
+
+For PhysioNet databases that require credentialed access, you can pass
+credentials via ``fsspec`` storage options or configure them in your
+environment before calling ``wfdb`` functions.
+
+
 Downloading
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