Add "beta" documentation for enabling object spilling manually (#13047)

doc/source/advanced.rst

@@ -392,3 +392,39 @@ To get information about the current available resource capacity of your cluster
 
 .. autofunction:: ray.available_resources
     :noindex:
+
+Object Spilling
+---------------
+
+Ray 1.2.0+ has *beta* support for spilling objects to external storage once the capacity
+of the object store is used up. Please file a `GitHub issue <https://github.com/ray-project/ray/issues/>`__
+if you encounter any problems with this new feature. Eventually, object spilling will be
+enabled by default, but for now you need to enable it manually:
+
+To enable object spilling to the local filesystem (single node clusters only):
+
+.. code-block:: python
+
+    ray.init(
+        _system_config={
+            "automatic_object_spilling_enabled": True,
+            "object_spilling_config": json.dumps(
+                {"type": "filesystem", "params": {"directory_path": "/tmp/spill"}},
+            )
+        },
+    )
+
+To enable object spilling to remote storage (any URI supported by `smart_open <https://pypi.org/project/smart-open/>`__):
+
+.. code-block:: python
+
+    ray.init(
+        _system_config={
+            "automatic_object_spilling_enabled": True,
+            "max_io_workers": 4,  # More IO workers for remote storage.
+            "min_spilling_size": 100 * 1024 * 1024,  # Spill at least 100MB at a time.
+            "object_spilling_config": json.dumps(
+                {"type": "smart_open", "params": {"uri": "s3:///bucket/path"}},
+            )
+        },
+    )

doc/source/walkthrough.rst

@@ -417,6 +417,8 @@ actors.
     to the object ref returned by the put exists. This only applies to the specific
     ref returned by put, not refs in general or copies of that refs.
 
+See also: `object spilling <advanced.html#object-spilling>`__.
+
 Remote Classes (Actors)
 -----------------------