clarify that the abstract store api is not normative

Author: jhamman

docs/v3/core/v3.0.rst

@@ -1273,6 +1273,8 @@ when opening a Zarr hierarchy to automatically determine the appropriate store.
 Abstract store interface
 ------------------------
 
+(This subsection is not normative.)
+
 The store interface is intended to be simple to implement using a
 variety of different underlying storage technologies. It is defined in
 a general way here, but it should be straightforward to translate into