Surface documentation for debug logging of system execution order

## What problem does this solve or what need does it fill?

System evaluation order in Bevy is deliberately nondeterministic if unspecified.

This can be quite challenging to debug.

## What solution would you like?

Add opt-in `info!` level logging to the [parallel executor](https://github.com/bevyengine/bevy/blob/main/crates/bevy_ecs/src/schedule/executor_parallel.rs).

This should probably be controlled via a feature flag.

## What alternative(s) have you considered?

Add logging statements to the systems of interest. This doesn't work effectively when the systems in question are external.