Italicize type-only values in `Returns` section

[LeanderBlume]

type-only values as in

Returns
-------
int

should be in italics for consistency with the parameter_name : int style, where int would be in italics.

Possible reason not to do this: "type-only" is just a convention and it could also be "name-only" and numpydoc wants to stay agnostic on this. If this is the case, feel free to close.