Improve to_zarr docs (#9139)

Promoted `region='auto'`, refine the langague slightly

xarray/core/dataset.py

@@ -2458,24 +2458,26 @@ def to_zarr(
             If set, the dimension along which the data will be appended. All
             other dimensions on overridden variables must remain the same size.
         region : dict or "auto", optional
-            Optional mapping from dimension names to integer slices along
-            dataset dimensions to indicate the region of existing zarr array(s)
-            in which to write this dataset's data. For example,
-            ``{'x': slice(0, 1000), 'y': slice(10000, 11000)}`` would indicate
-            that values should be written to the region ``0:1000`` along ``x``
-            and ``10000:11000`` along ``y``.
-
-            Can also specify ``"auto"``, in which case the existing store will be
-            opened and the region inferred by matching the new data's coordinates.
-            ``"auto"`` can be used as a single string, which will automatically infer
-            the region for all dimensions, or as dictionary values for specific
-            dimensions mixed together with explicit slices for other dimensions.
+            Optional mapping from dimension names to either a) ``"auto"``, or b) integer
+            slices, indicating the region of existing zarr array(s) in which to write
+            this dataset's data.
+
+            If ``"auto"`` is provided the existing store will be opened and the region
+            inferred by matching indexes. ``"auto"`` can be used as a single string,
+            which will automatically infer the region for all dimensions, or as
+            dictionary values for specific dimensions mixed together with explicit
+            slices for other dimensions.
+
+            Alternatively integer slices can be provided; for example, ``{'x': slice(0,
+            1000), 'y': slice(10000, 11000)}`` would indicate that values should be
+            written to the region ``0:1000`` along ``x`` and ``10000:11000`` along
+            ``y``.
 
             Two restrictions apply to the use of ``region``:
 
             - If ``region`` is set, _all_ variables in a dataset must have at
               least one dimension in common with the region. Other variables
-              should be written in a separate call to ``to_zarr()``.
+              should be written in a separate single call to ``to_zarr()``.
             - Dimensions cannot be included in both ``region`` and
               ``append_dim`` at the same time. To create empty arrays to fill
               in with ``region``, use a separate call to ``to_zarr()`` with