DOC: update DatetimeTZDtype unit limitation (pandas-dev#58213)

* doc: update DatetimeTZDtype unit limitation

* fix description

Author: yokomotod

pandas/core/dtypes/dtypes.py

@@ -698,8 +698,8 @@ class DatetimeTZDtype(PandasExtensionDtype):
     Parameters
     ----------
     unit : str, default "ns"
-        The precision of the datetime data. Currently limited
-        to ``"ns"``.
+        The precision of the datetime data. Valid options are
+        ``"s"``, ``"ms"``, ``"us"``, ``"ns"``.
     tz : str, int, or datetime.tzinfo
         The timezone.