Document ScheduleLabel dependencies

## How can Bevy's documentation be improved?

The [ScheduleLabel derive macro](https://docs.rs/bevy/latest/bevy/ecs/schedule/derive.ScheduleLabel.html) requires that a type additionally implement `Hash, Debug, Eq, PartialEq, Clone`, but this is only evident when attempting and failing to compile a type missing these traits.

Start with the ScheduleLabel derive macro documentation linked in the issue and inspect how its requirements are currently presented. Document that derived types must implement Hash, Debug, Eq, PartialEq, and Clone; done means the requirements are visible without needing to compile a failing example.

Written by the indexing model from the issue text.