DOC: Add note to docs on DataTree root-group naming conventions (#9298)

* DOC: Add note to docs on DataTree root-group naming conventions (root group name is always "/").

* Update xarray/core/datatree.py

* Update xarray/core/datatree.py

Co-authored-by: Matt Savoie <github@flamingbear.com>

* Update xarray/core/datatree.py

Co-authored-by: Matt Savoie <github@flamingbear.com>

* Update xarray/datatree_/docs/source/io.rst

Co-authored-by: Matt Savoie <github@flamingbear.com>

* Update xarray/datatree_/docs/source/io.rst

Co-authored-by: Matt Savoie <github@flamingbear.com>

---------

Co-authored-by: Matt Savoie <github@flamingbear.com>

Author: kmuehlbauer

xarray/core/datatree.py

@@ -1551,7 +1551,12 @@ def to_netcdf(
             ``dask.delayed.Delayed`` object that can be computed later.
             Currently, ``compute=False`` is not supported.
         kwargs :
-            Addional keyword arguments to be passed to ``xarray.Dataset.to_netcdf``
+            Additional keyword arguments to be passed to ``xarray.Dataset.to_netcdf``
+
+        Note
+        ----
+            Due to file format specifications the on-disk root group name
+            is always ``"/"`` overriding any given ``DataTree`` root node name.
         """
         from xarray.core.datatree_io import _datatree_to_netcdf
 
@@ -1607,6 +1612,11 @@ def to_zarr(
             supported.
         kwargs :
             Additional keyword arguments to be passed to ``xarray.Dataset.to_zarr``
+
+        Note
+        ----
+            Due to file format specifications the on-disk root group name
+            is always ``"/"`` overriding any given ``DataTree`` root node name.
         """
         from xarray.core.datatree_io import _datatree_to_zarr
 

xarray/datatree_/docs/source/io.rst

@@ -24,6 +24,12 @@ To open a whole netCDF file as a tree of groups use the :py:func:`open_datatree`
 To save a DataTree object as a netCDF file containing many groups, use the :py:meth:`DataTree.to_netcdf` method.
 
 
+.. _netcdf.root_group.note:
+
+.. note::
+    Due to file format specifications the on-disk root group name is always ``"/"``,
+    overriding any given ``DataTree`` root node name.
+
 .. _netcdf.group.warning:
 
 .. warning::
@@ -52,3 +58,8 @@ To save a DataTree object as a zarr store containing many groups, use the :py:me
 .. note::
     Note that perfect round-tripping should always be possible with a zarr store (:ref:`unlike for netCDF files <netcdf.group.warning>`),
     as zarr does not support "unused" dimensions.
+
+    For the root group the same restrictions (:ref:`as for netCDF files <netcdf.root_group.note>`) apply.
+    Due to file format specifications the on-disk root group name is always ``"/"``
+    overriding any given ``DataTree`` root node name.
+