Update docs on using until() in attribute interpolations

[justinfagnani]

We've just introduced a new, hopefully minor and rarely hit, breaking change with how until() operates. Previously when until() was used in an interpolated attribute binding it would prevent the attribute from being rendered at all until there was an initial value. Now the attribute is rendered with an empty string where the until() occurs.

The new behavior is as if you wrote until(promise, ''). To get the old behavior you need to opt-in with nothing: until(promise, nothing).