DOCSP-47916: Add extra info to Decimal128 data type (#33)

Author: mcmorisi

source/data-types.txt

@@ -51,6 +51,7 @@ Support for additional types will be added in subsequent releases.
      - - ``bson.Decimal128``
        - Instance of ``pymongoarrow.types.Decimal128Type``
        - Instance of ``pymongoarrow.pandas_types.PandasDecimal128``
+       - Instance of ``pyarrow.Decimal128``
    * - Boolean
      - - ``~py.bool``
        - Instance of ``~pyarrow.bool_``