docs: document ALTER KEYSPACE with tablets

docs/cql/ddl.rst

@@ -289,6 +289,17 @@ For instance::
 
 The supported options are the same as :ref:`creating a keyspace <create-keyspace-statement>`.
 
+ALTER KEYSPACE with Tablets :label-caution:`Experimental`
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Modifying a keyspace with tablets enabled is possible and doesn't require any special CQL syntax. However, there are some limitations:
+
+- The replication factor (RF) can be increased or decreased by at most 1 at a time. To reach the desired RF value, modify the RF repeatedly.
+- The ``ALTER`` statement rejects the ``replication_factor`` tag. List the DCs explicitly when altering a keyspace. See :ref:`NetworkTopologyStrategy <replication-strategy>`.
+- If there's any other ongoing global topology operation, executing the ``ALTER`` statement will fail (with an explicit and specific error) and needs to be repeated.
+- The ``ALTER`` statement may take longer than the regular query timeout, and even if it times out, it will continue to execute in the background.
+- The replication strategy cannot be modified, as keyspaces with tablets only support ``NetworkTopologyStrategy``.
+
 .. _drop-keyspace-statement:
 
 DROP KEYSPACE

docs/operating-scylla/procedures/cluster-management/add-dc-to-existing-dc.rst

@@ -156,7 +156,9 @@ Add New DC
       UN   54.160.174.243  109.54 KB       256     ?               c7686ffd-7a5b-4124-858e-df2e61130aaa    RACK1
       UN   54.235.9.159    109.75 KB       256     ?               39798227-9f6f-4868-8193-08570856c09a    RACK1
       UN   54.146.228.25   128.33 KB       256     ?               7a4957a1-9590-4434-9746-9c8a6f796a0c    RACK1
-   
+
+.. TODO possibly provide additional information WRT how ALTER works with tablets
+
 #. When all nodes are up and running ``ALTER`` the following Keyspaces in the new nodes:
 
    * Keyspace created by the user (which needed to replicate to the new DC).