Improve the -1 docs / handling in core

[brendanheywood]

Either:

1. Have a magic constant so its it's -1 then then don't show the ratio on the page
   OR
2. we fix core so this value can be 0 and it still works

[bwalkerl]

I've included the change to fix the core setting as part of MDL-73798 - logically it doesn't make sense to use the default bounce ratio if you've explicitly set something.

There's still room to improve the docs to mention the expectation so I'll leave this open for now.