Add brief documentation for the ProtoRedundantSet bugpattern.
Add brief documentation for the ProtoRedundantSet bugpattern.

RELNOTES: N/A

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=196228719

cushon/error-prone,cushon/error-prone,cushon/error-prone,google/error-prone,cushon/error-prone,google/error-prone