DOC: State that we support scalars in to_numeric (pandas-dev#24944)

We support it and test it already.

xref pandas-devgh-24910.

Author: gfyoung

pandas/core/tools/numeric.py

@@ -21,7 +21,7 @@ def to_numeric(arg, errors='raise', downcast=None):
 
     Parameters
     ----------
-    arg : list, tuple, 1-d array, or Series
+    arg : scalar, list, tuple, 1-d array, or Series
     errors : {'ignore', 'raise', 'coerce'}, default 'raise'
         - If 'raise', then invalid parsing will raise an exception
         - If 'coerce', then invalid parsing will be set as NaN