DOC: Clarify to_numeric behavior for numeric dtypes (#61904)

Author: chilin0525

pandas/core/tools/numeric.py

@@ -55,7 +55,8 @@ 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.