Details omitted in `Watcher.getPending()`

[prophile]

The specification for Signal.subtle.Watcher.prototype.getPending() specifies:

Return an Array containing the subset of signals which are in the state dirty or pending.

This should probably:

1. Read ~dirty~ and ~pending~,
2. Specify the actual order these will be returned explicitly,
3. Say something about callers mutating the return value, even if it's that the behaviour is explicitly undefined.