E.sendOnly(e).foo(args), etc, should have void return type

[erights]

endo/packages/eventual-send/src/E.js

Line 299 in 7a29550

* @typedef {(...args: Parameters<T>) => Promise<void>} ESendOnlyCallable

says it returns a promise for void. But does it actually? AFAICT, it immediately returns only undefined and therefore should be declared as returning either void or undefined.

I noticed this when I got a @typescript-eslint/no-floating-promises lint error on the statement

 E.sendOnly(guestPingee).ping('sendOnly');

[erights]

@michaelfig , same problem at https://github.com/Agoric/agoric-sdk/blob/cbe061cdb847d99ea435c26131709527b40a1ce5/packages/vow/src/E.js#L354 ?