Clarify that key_suffix is not scoped to each strategy

[CJStadler]

This PR updates the README example showing multiple concurrency strategies to make sure they generate different key suffixes. Previously the example was misleading because the calculated key suffixes could overlap, causing surprising behavior.

The example says it is intended to

Allow maximum 10 concurrent jobs per project at a time and maximum 2 jobs per user

But if there were two jobs running with project_id 1 then all jobs for user_id 1 would also be throttled, because there would already be two jobs under the key 1. This is probably not the intended behavior.

This PR also adds a spec to demonstrate the behavior.

@ixti do you have any other suggestions on how to clarify this behavior? Or do you think it should be considered a bug?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.97%. Comparing base (854dffe) to head (259670b).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

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  Coverage   98.97%   98.97%           
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  Files          19       19           
  Lines         486      486           
  Branches       85       85           
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  Hits          481      481           
  Misses          5        5           

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[ixti]

The documentation was wrong, indeed. Thank you for the PR and the fix.