Documentation of urgency values differs from actual urgency values

[claremacrae]

In #711 I learned that the currently-calculated urgency values differ from what is documented.

See the documentation says comments in this snippet from the tests:

https://github.com/obsidian-tasks-group/obsidian-tasks/blob/0741ac11cbfbb13cbf28a6e5ecdc1f5d10bbbd04/tests/Query/Urgency.test.ts#L82-L95

More thought is needed to decide how or whether to change the urgency calculation, but for now, I feel the documentation should be updated to match the current behaviour.