Update release note

Author: dstansby

docs/release.rst

@@ -28,6 +28,8 @@ Fixes
 * Fixes issue with ``Delta`` Zarr 3 codec not working with ``astype``.
   By :user:`Norman Rzepka <normanrz>`, :issue:`664`
 * Cleanup ``PCodec`` soft dependency.
+  Previously importing ``numcodecs.pcodec`` would work if ``pcodec`` is not installed,
+  but now it will fail to import. This mirrors the behaviour of other optional dependencies.
   By :user:`John Kirkham <jakirkham>`, :issue:`647`