Doc: Clarify to_numeric behavior for numeric dtypes

Author: chilin0525

pandas/core/tools/numeric.py

@@ -55,9 +55,11 @@ def to_numeric(
     """
     Convert argument to a numeric type.
 
-    The default return dtype is `float64` or `int64`
+    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.
+    to obtain other dtypes. Numeric dtypes include all dtypes that have
+    the `_is_numeric` attribute set to `True`.
 
     Please note that precision loss may occur if really large numbers
     are passed in. Due to the internal limitations of `ndarray`, if