RELNOTES: Updating UnnecessaryDefaultEnumSwitch docs to explain UNRECOGNIZED.
RELNOTES: Updating UnnecessaryDefaultEnumSwitch docs to explain UNRECOGNIZED.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=205935122

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