remove internal attribute from doc

Author: chilin0525

pandas/core/tools/numeric.py

@@ -58,8 +58,7 @@ def to_numeric(
     If the input is already of a numeric dtype, the dtype will be preserved.
     For non-numeric inputs, the default return dtype is `float64` or `int64`
     depending on the data supplied. Use the `downcast` parameter
-    to obtain other dtypes. Numeric dtypes include all dtypes that have
-    the `_is_numeric` attribute set to `True`.
+    to obtain other dtypes.
 
     Please note that precision loss may occur if really large numbers
     are passed in. Due to the internal limitations of `ndarray`, if