doc: add comment explaining recentRejects-DoS behavior
doc: add comment explaining recentRejects-DoS behavior

When we receive invalid txs for the first time, we mark the sender as
misbehaving. If we receive the same tx before a new block is seen, we *don't*
punish the second sender (in the same way we do the original sender). It wasn't
initially clear to me that this is intentional, so add a clarifying comment.

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