Explain `'w` and `'s` lifetimes better in documentation

## How can Bevy's documentation be improved?

There are a lot of types in `bevy_ecs` that has the `'w` and `'s` lifetimes:

- `EventReader`
- `EventWriter`
- `QueryIter`
- `QueryManyIter`
- `QueryCombinationIter`
- `SystemParamItem`
- `ParamSet`
- `Commands`
- `EntityCommands`
- and more...

The best explanation found for these lifetimes are in [`SystemParam`](https://docs.rs/bevy/latest/bevy/ecs/system/trait.SystemParam.html#derive)

Solutions I could think of right now:

1. Rename `'w` and `'s` to `'world` and `'state`. This is not ideal since this will make the code more verbose.
2. Link users to the `SystemParam` documentation. There are a lot of types to document and imo would add more noise to their documentation.
3. Put the documentation in the README, perhaps under a new "Advanced topics" sections.