[optional.ref.assign] Add missing Returns element

[eisenwave]

Notice that the return type is T&, but convert-ref-init-val is a void member function.

The intent is obviously to return *val; this seems like an editorial slip-up (unfortunately resulting in a normative defect).

[jwakely]

We'd usually just say Returns: *val in cases like this.

[jwakely]

i.e. nothing in the current wording says it returns the result of convert-ref-init-val so the fact it's a void expression is irrelevant.

[eisenwave]

You will be shocked to find out what we've just merged is exactly the solution in LWG4300, which was just approved in Kona.

[jwakely]

We'd usually just say Returns: *val in cases like this.

Well at least I was consistent 😅