ZEP9 (phase 1): add clarifications for extension naming

docs/v3/array-storage-transformers.rst

@@ -2,12 +2,20 @@
 Array Storage Transformers
 ==========================
 
-Under construction.
+.. COMMENT TO BE REMOVED WHEN ONE IS ADDED
 
-.. toctree::
+   The following documents specify transformers which are defined by the maintainers of
+   the Zarr specification. Being listed below does not imply that a transformer is
+   required to be implemented by implementations.
+
+   toctree::
    :glob:
    :maxdepth: 1
    :titlesonly:
    :caption: Contents:
 
    array-storage-transformers/*/*
+
+Currently, no transformers are defined by the maintainers of
+the Zarr specification. Being listed below does not imply that a transformer is
+required to be implemented by implementations.

docs/v3/codecs.rst

@@ -2,7 +2,9 @@
 Codecs
 ======
 
-Under construction.
+The following documents specify codecs which are defined by the maintainers
+of the Zarr specification. Being listed below does not imply that a codec is
+required to be implemented by all implementations.
 
 .. toctree::
    :glob: