[Docs] Mention how to skip subsets of exceptions in errorpages.rst
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It appears that skipping subsets of exceptions in a generic error handler has been a major point of confusion; there are at least three open issues related to this: #2778, #2841, #2984.
This PR documents the workaround without modifying current behavior (whether 3XX should be captured seems to be a point of contention in the aforementioned issues).