Document and decide on arithmetic toPrimitive footguns

[sffc]

We should better document the consequences of toPrimitive and consider how it impacts call sites such as

new Amount(5) + 5
new Amount(5) * 5
new Amount(5)++

We may want to reconsider whether any of these are too surprising.

[jessealama]

Just to document for myself what happens in various currently-existing scenarios:

let a = "1.23";
a++;
a; // 2.23 (String converted to Number)

let b = "1.23[kg]";
b++;
b; // NaN, got converted to Number

let c = { "value": "1.23", "unit": "kg" };
c++;
c; // NaN, got converted to Number