add placeholder disclaimer to v3 data types summary

Author: d-v-b

docs/user-guide/data_types.rst

@@ -45,7 +45,7 @@ as are floats, with the caveat that `NaN`, positive infinity, and negative infin
 
 Data types in Zarr version 3
 ----------------------------
-
+(note: placeholder text)
 * Data type names are different -- Zarr V2 represented the 16 bit unsigned integer data type as ``>i2``; Zarr V3 represents the same data type as ``int16``.
 * No endianness
 * A data type can be encoded in metadata as a string or a ``JSON`` object with the structure ``{"name": <string identifier>, "configuration": {...}}``