Document that with methods accept Temporal object instances

[justingrant]

Because in #403 it was decided that fields shouldn't be enumerable, I assumed that code like the following was required to use with with Temporal object instances.

const dt = Temporal.now.dateTime();
const midnight = Temporal.Time.from('00:00');
const newYear = Temporal.MonthDay.from({month: 1, day: 1});

const todayMidnight = dt.with({...midnight.getFields()});
const thisJan1 = dt.with({...newYear.getFields()});

This assumption was wrong, which seems obvious now. But it still took me a few weeks to realize that I can use this much easier and intuitive code:

// much easier!
const todayMidnight = dt.with(midnight);
const thisJan1 = dt.with(newYear);

// the same trick works with `from` too!  (this is already documented though)
Temporal.DateTime.from(Temporal.now.date())
Temporal.Time.from(Temporal.now.dateTime())

In a quick look at the docs for with, I only saw example code with object literals, which made this trick harder to discover. I think I'd slotted these methods into a mental box that said "only accepts object literals". This is obviously wrong in retrospect but is a good example of how unintended messages in the docs can persist in novice developers' heads.

IMHO, it's worth adding additional samples to documentation for with to illustrate that even though you can't spread a Temporal instance into an object, you can pass it into with of another compatible Temporal instance.

[ptomato]

I think the cases that are worth documenting are:

• Date.with(YearMonth)
• Date.with(MonthDay)
• DateTime.with(Time)
• DateTime.with(Date)
• DateTime.with(YearMonth)
• DateTime.with(MonthDay)

Anything else, we should probably not be encouraging, as it would overwrite all the fields and should probably better use from() or one of the type conversion methods.