update release note (#3230)

Author: d-v-b

changes/2819.chore.rst

@@ -1,4 +1,18 @@
 Ensure that invocations of ``create_array`` use consistent keyword arguments, with consistent defaults.
-Specifically, ``zarr.api.synchronous.create_array`` now takes a ``write_data`` keyword argument; The
-``create_array`` method on ``zarr.Group`` takes ``data`` and ``write_data`` keyword arguments. The ``fill_value``
-keyword argument of the various invocations of ``create_array`` has been consistently set to ``None``, where previously it was either ``None`` or ``0``.
+
+- ``zarr.api.synchronous.create_array`` now takes a ``write_data`` keyword argument
+- The ``Group.create_array`` method takes ``data`` and ``write_data`` keyword arguments.
+- The functions ``api.asynchronous.create``, ``api.asynchronous.create_array``
+  and the methods ``Group.create_array``, ``Group.array``, had the default
+  ``fill_value`` changed from ``0`` to the ``DEFAULT_FILL_VALUE`` value, which instructs Zarr to
+  use the default scalar value associated with the array's data type as the fill value. These are
+  all functions or methods for array creation that mirror, wrap or are wrapped by, another function
+  that already has a default ``fill_value`` set to ``DEFAULT_FILL_VALUE``. This change is necessary
+  to make these functions consistent across the entire codebase, but as this changes default values,
+  new data might have a different fill value than expected after this change.
+
+  For data types where 0 is meaningful, like integers or floats, the default scalar is 0, so this
+  change should not be noticeable. For data types where 0 is ambiguous, like fixed-length unicode
+  strings, the default fill value might be different after this change. Users who were relying on how
+  Zarr interpreted ``0`` as a non-numeric scalar value should set their desired fill value explicitly
+  after this change.