Add docs about using string keys to create multiple limits per unit of time.

[jakeonrails]

Add docs about using string keys to create multiple limits per unit of time.

I was confused about implementing this type of behavior until I saw your comment in #149, explaining the use of strings for the key_suffix. Hopefully this doc will help other realize they don't need a proc to do this but that the key_suffix is required for having multiple limits.

I was simply doing this:

throttle: [
  { limit: 500, period: 1.minute },
  { limit: 18_000, period: 1.day }
]

This showed up in the UI on Sidekiq Admin as two separate throttles so it looked like it worked but it does not.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.71%. Comparing base (a15db1b) to head (c65f387).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #202   +/-   ##
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  Coverage   98.71%   98.71%           
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  Files          19       19           
  Lines         466      466           
  Branches       77       77           
=======================================
  Hits          460      460           
  Misses          6        6           

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