Document some phpdoc return type changes in OCP in 27

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

Author: come-nc

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_27.rst

@@ -58,6 +58,12 @@ Removed APIs
 
 * Intermediate transition event classes ``\OCP\WorkflowEngine\IEntityCompat`` and ``\OCP\WorkflowEngine\IOperationCompat`` have been removed as advertised for 2023 (`nextcloud/server#37040 <https://github.com/nextcloud/server/pull/37040>`_)
 
+Changed APIs
+^^^^^^^^^^^^
+
+* ``\OCP\Files\FileInfo::getOwner`` documented return type is now nullable, to match what was already returned by the implementation (`nextcloud/server#36836 <https://github.com/nextcloud/server/pull/36836>`_)
+* ``\OCP\Files\File::fopen`` and ``\OCP\Files\SimpleFS\ISimpleFile::read`` documented return types are now nullable, to match what was already returned by the implementations (`nextcloud/server#36836 <https://github.com/nextcloud/server/pull/36836>`_)
+
 
 Behavioral changes
 ^^^^^^^^^^^^^^^^^^