`SystemState` can be cached using its `ExclusiveSystemParam` implementation, but the docs don't mention this.

## How can Bevy's documentation be improved?

This issue is a more specific form of #8770, which has been open for over nine months with no apparent resolution.

The [caching example] for `SystemState` in the documentation shows the use of a function-local struct to cache a `SystemState` object. However, caching a `SystemState` has gotten easier since this documentation was written - with the addition of `ExclusiveSystemParam` and its implementation on `&'a mut SystemState

`, we can have the ECS handle caching for us.

The documentation should be updated to reflect this.

[caching example]: https://docs.rs/bevy/latest/bevy/ecs/system/struct.SystemState.html#example

Start with the SystemState API documentation and its linked caching example on docs.rs, then read the surrounding documentation for ExclusiveSystemParam. Update the example to reflect ECS-managed caching through the &mut SystemState implementation, and verify that the documentation builds and clearly shows the revised usage.

Written by the indexing model from the issue text.