Updating the docs to specifically call out some common error messages about semian HOT 3 CLOSED

Regarding timed out waiting for resource, it seems that for sysv semaphores, this means that the request to set the semaphore has failed. Since we're in a non-gvl area, it seems entirely possible that a multi-threaded or multi-process application could be trying to access the semaphore system at the same time, causing the EAGAIN. This doesn't seem catastrophic, so I'm going to try to rescue this error and retry, then if that seems to go okay, I'll see if I can write some code that adds in retries. I now think that this is actually what's going on when the Bulkhead is full. I don't understand why sometimes we get a ResourceBusyError and sometimes we get this error message, but I no longer thing that a retry would do anything.

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Regarding execution expired, Semian seems to only output the last_error.message, so it's unclear what this could be. It could be from Net::OpenTimeout, or a bunch of other classes. I'll post a PR in a bit to include the class name in the exception message and see what the maintainers think. (This was already done, but I was behind in gem versions. Thank you!)

Edit: After much searching, I believe this to almost always be a case of Net::*Timeout (for those externals that are using net/http).

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Since I can't figure out the exact semantics of this, I'm going to close this issue without updating the docs. I don't want to lead people astray unless I feel much more confident about what these errors mean!

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