doc: update datetime64[ns] to datetime64[us] for pandas version >= 2.0.0

Author: kitagry

docs/reading.rst

@@ -56,17 +56,17 @@ Inferring the DataFrame's dtypes
 The :func:`~pandas_gbq.read_gbq` method infers the pandas dtype for each
 column, based on the BigQuery table schema.
 
-================== =========================
+================== ============================================
 BigQuery Data Type dtype
-================== =========================
+================== ============================================
 BOOL               boolean
 INT64              Int64
 FLOAT64            float64
 TIME               dbtime
 DATE               dbdate or object
-DATETIME           datetime64[ns] or object
-TIMESTAMP          datetime64[ns, UTC] or object
-================== =========================
+DATETIME           datetime64[ns] (datetime64[us] if pandas version >= 2.0.0) or object
+TIMESTAMP          datetime64[ns, UTC] (datetime64[us, UTC] if pandas version >= 2.0.0) or object
+================== ============================================
 
 If any DATE/DATETIME/TIMESTAMP value is outside of the range of `pandas.Timestamp.min
 <https://pandas.pydata.org/docs/reference/api/pandas.Timestamp.min.html>`__