feat(dev): Document return and param types changes for OCP\Files\Storage\IStorage inheritors in 31

Signed-off-by: provokateurin <kate@provokateurin.de>

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_31.rst

@@ -76,6 +76,11 @@ Changed APIs
 ^^^^^^^^^^^^
 
 - Clarify ``OCP\Files\Storage\IStorage::getOwner()`` returns ``string|false``.
+- Added method parameter and return types to all inheritors of ``OCP\Files\Storage\IStorage``. To migrate in a backwards compatible manner:
+
+  #. Add all return types to your implementation now.
+  #. Add all parameter types to your implementation once Nextcloud 31 is the lowest supported version.
+
 - The Nextcloud implementation of the ``log`` method of ``Psr\Log\LoggerInterface`` now supports ``Psr\Log\LogLevel`` as log level parameter.
 
 Deprecated APIs
@@ -98,4 +103,4 @@ Removed APIs
     - The ``OCP\ILogger`` interface can no longer be dependency injected as it now only holds constants.
     - ``OCP\IServerContainer::getLogger`` was removed, use dependency injection with ``Psr\Log\LoggerInterface`` instead.
 - The internal class ``OC\AppFramework\Logger`` was removed, it should have been never used by apps.
-  All using apps should migrate to ``Psr\Log\LoggerInterface``.
+  All using apps should migrate to ``Psr\Log\LoggerInterface``.