docs(ai/assistant): document the assistant data folder name

The assistant creates a folder in each user's files for the content it
generates. Its name is now configurable server-wide and per user, but neither
the key nor the behaviour around existing folders was written down anywhere.

Documents both settings next to the other assistant occ options, and the two
things that surprise people: changing the name leaves existing folders alone,
and a user who already has a folder under the built-in name keeps it rather
than getting a second one.

Signed-off-by: Baki Burak Öğün <63836730+bakiburakogun@users.noreply.github.com>

Author: bakiburakogun

admin_manual/ai/app_assistant.rst

@@ -190,6 +190,33 @@ To enable/disable the text-to-image smart picker for all the users.
 
 To enable/disable the speech-to-text smart picker for all the users.
 
+5. Data folder
+
+.. code-block::
+
+   occ config:app:set assistant default_data_folder --value="Assistant" --type=string
+
+The assistant stores the content it generates, such as generated images and speech-to-text
+output, in a folder in the files of each user. This sets the name that folder is given when
+it is created. It defaults to ``Assistant``, and is also editable in the Assistant admin
+settings.
+
+Users can choose their own name in their personal Assistant settings, which is stored per
+user and takes precedence over the server-wide default:
+
+.. code-block::
+
+   occ user:setting $USER_ID assistant data_folder "Assistant"
+
+Two things to keep in mind:
+
+* Changing either value does not rename or move a folder that already exists. It only
+  affects the name used the next time a folder has to be created, so existing generated
+  content stays where it is.
+* A user who already has a folder under the built-in name ``Assistant``, from before a
+  default was set, keeps using it. This avoids starting a second folder for them and
+  leaving their earlier output behind.
+
 Task processing
 ~~~~~~~~~~~~~~~