The example for `when` doesn't show that it takes the condition value as a parameter

The when directive docs are out of date with the actual v3 API. Since lit#4310, when() passes the (type-narrowed) condition value to both case functions, but the docs don't reflect this.

1. The Signature box is wrong

It currently shows:

when<T, F>(
  condition: boolean,
  trueCase: () => T,
  falseCase?: () => F
)

The actual v3 signature uses a generic condition and passes it to the callbacks:

when<C, T, F = undefined>(
  condition: C,
  trueCase: (c: Exclude<C, Falsy>) => T,
  falseCase?: (c: Extract<C, Falsy>) => F
): C extends Falsy ? F : T

(We may want to simplify it for the docs? I'm not so sure)

2. The example doesn't use the parameter

It currently is:

${when(this.user, () => html`User: ${this.user.username}`, () => html`Sign In...`)}

It should show the value being passed in (and narrowed), which is the whole point of the feature:

${when(this.user, (user) => html`User: ${user.username}`, () => html`Sign In...`)}

Additional note

The same incomplete example is also in the upstream JSDoc at lit/packages/lit-html/src/directives/when.ts, so that may be worth updating too for consistency.

Start with the when directive documentation linked in the issue and compare its Signature box and example with the v3 API described here. Then inspect lit/packages/lit-html/src/directives/when.ts for the upstream JSDoc example. Done means both examples show the condition value being passed and the documentation signature accurately reflects the current API.