While trying to figure out how to implement a `SystemParam`, I spent a
long time looking for a feature that would do exactly what `Config`
does. I ignored it at first because all the examples I could find used
`()` and I couldn't see a way to modify it.
This is documented in other places, but `Config` is a logical place to
include some breadcrumbs. I've added some text that gives a brief
overview of what `Config` is for, and links to the existing docs on
`FunctionSystem::config` for more details.
This would have saved me from embarrassing myself by filing https://github.com/bevyengine/bevy/issues/2178.
Co-authored-by: Carter Anderson <mcanders1@gmail.com>
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