Mention that ProtoFieldNullComparison does not apply to nano protos.
Mention that ProtoFieldNullComparison does not apply to nano protos.

And add an example of another case it matches.

RELNOTES: N/A

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=214563229

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