Merge #14436: doc: add comment explaining recentRejects-DoS behavior

Summary:
b191c7dfb7 doc: add comment explaining recentRejects-DoS behavior (James O'Beirne)

Pull request description:

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.

Tree-SHA512: d12c674db137ed3ad83e0b941bffe6ddcd2982238048742afa574a4235881f0e58cfc0a4a576a0503e74c5c5240c270b9520fa30221e8b43a371fb3e0b37066b

Backport of Core PR14436

Test Plan:

ninja

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6457