update reactivity docs, and add tests

[koculu]

Reactive behavior of single-prop bindings:

1. If the binding expression resolves to a ref, the child receives a live reactive prop channel.
2. If the binding expression resolves to a non-ref value, the child receives the current resolved value.

Example:

<Btn :disabled="busy || !pendingDeleteHostname"></Btn>

This passes the current boolean result.

If you want the computed prop expression itself to stay reactive as a component
input, wrap it with ref(...):

<Btn :disabled="ref(busy || !pendingDeleteHostname)"></Btn>