Update description of RegistrarCredit in g3doc architecture doc
Update description of RegistrarCredit in g3doc architecture doc

This is a followup to [] that adds a similar explanation to our g3doc, warning that RegistrarCredits don't have any effect as implemented and may be removed.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=151041925

google/nomulus,google/nomulus,google/nomulus,google/nomulus,google/nomulus,google/nomulus