feat: Document IAppManager::getAppNamespace new method

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

Author: come-nc

developer_manual/release_notes/deprecations.rst

@@ -20,6 +20,12 @@ New deprecations
 
 .. todo:: This page needs a section for every new deprecation.
 
+Application management
+^^^^^^^^^^^^^^^^^^^^^^
+
+- ``\OCP\AppFramework\App::buildAppNamespace`` is deprecated in favor of non-static method ``\OCP\App\IAppManager::getAppNamespace``
+
+
 Older deprecations
 ------------------
 

developer_manual/release_notes/new.rst

@@ -34,3 +34,12 @@ but they might be required to have a fully working instance later on.
 Expensive repair steps are only executed when explicitly requested by the administrator.
 
 See :ref:`migration-repair-steps` for details.
+
+Application namespace management
+--------------------------------
+
+``\OCP\App\IAppManager`` was extended with two new methods related to application namespaces:
+- ``getAppNamespace(string $appId): string`` returns the namespace for an application from its appid
+- ``getAppFromNamespace(string $className): ?string`` does the opposite. Less common but it is used in guests application.
+
+This replaces a static method in ``\OCP\AppFramework\App`` which is now deprecated.