add docstrings

Author: TrevorBergeron

bigframes/dataframe.py

@@ -1309,6 +1309,8 @@ def __neg__(self) -> DataFrame:
     def __abs__(self) -> DataFrame:
         return self._apply_unary_op(ops.abs_op)
 
+    __abs__.__doc__ = abs.__doc__
+
     def align(
         self,
         other: typing.Union[DataFrame, bigframes.series.Series],

bigframes/series.py

@@ -1363,6 +1363,8 @@ def update(self, other: Union[Series, Sequence, Mapping]) -> None:
     def __abs__(self) -> Series:
         return self.abs()
 
+    __abs__.__doc__ = inspect.getdoc(vendored_pandas_series.Series.abs)
+
     def abs(self) -> Series:
         return self._apply_unary_op(ops.abs_op)