docs: document the views-with-tablets experimental feature

Refs scylladb/scylladb#22217

Fixes scylladb/scylladb#22893

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

Closes scylladb/scylladb#22896

docs/architecture/tablets.rst

@@ -149,6 +149,15 @@ enabled:
 If you plan to use any of the above features, CREATE your keyspace
 :ref:`with tablets disabled <tablets-enable-tablets>`.
 
+The following ScyllaDB features are disabled by default when used with a keyspace
+that has tablets enabled:
+
+* Materialized Views (MV)
+* Secondary indexes (SI, as it depends on MV)
+
+To enable MV and SI for tablet keyspaces, use the `--experimental-features=views-with-tablets`
+configuration option.  See :ref:`Views with tablets <admin-views-with-tablets>` for details.
+
 Resharding in keyspaces with tablets enabled has the following limitations:
 
 * ScyllaDB does not support reducing the number of shards after node restart.

docs/operating-scylla/admin.rst

@@ -352,6 +352,24 @@ Before creating keyspaces with object storage, you also need to
 :ref:`configure <object-storage-configuration>` the object storage
 credentials and endpoint.
 
+.. _admin-views-with-tablets:
+
+Views with tablets
+------------------
+
+By default, Materialized Views (MV) and Secondary Indexes (SI)
+are disabled in keyspaces that use tablets.
+
+Support for MV and SI with tablets is experimental and must be explicitly
+enabled in the ``scylla.yaml`` configuration file by specifying
+the ``views-with-tablets`` option:
+
+.. code-block:: yaml
+
+   experimental_features:
+     - views-with-tablets
+
+
 Monitoring
 ==========
 ScyllaDB exposes interfaces for online monitoring, as described below.