Attempt to clarify docs how to use LimitedCapacity::Worker

What does this MR do and why?

The previous documentation said "a cron worker must be defined as well" which did not seem to be correct as some of these workers aren't queued this way. Additionally the previous example didn't make sense because cron workers don't get passed arguments.

I've also opted to remove the duplicated docs example from the class for a single source of truth. I also like the docs in the Ruby code to make it clearer to the reader but the risk of getting out of sync with the primary documentation (and the fact that it was currently confusing) seemed like it wasn't worth keeping.

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