Merge bitcoin/bitcoin#24310: docs / fixups from RBF and packages

77202f0554 [doc] package deduplication (glozow)
d35a3cb396 [doc] clarify inaccurate comment about replacements paying higher feerate (glozow)
5ae187f876 [validation] look up transaction by txid (glozow)

Pull request description:

  - Use txid, not wtxid, for `mempool.GetIter()`: https://github.com/bitcoin/bitcoin/pull/22674#discussion_r772934994
  - Fix a historically inaccurate comment about RBF during the refactors: https://github.com/bitcoin/bitcoin/pull/22855#discussion_r777130441
  - Add a section about package deduplication to policy/packages.md: https://github.com/bitcoin/bitcoin/pull/24152#discussion_r802955759 and https://github.com/bitcoin/bitcoin/pull/24152#discussion_r802723149

  (I'm intending for this to be in v23 since it's fixups for things that are already merged, which is why I split it from #24152)

ACKs for top commit:
  t-bast:
    LGTM, ACK 77202f0554
  darosior:
    ACK 77202f0554
  LarryRuane:
    ACK 77202f0554

Tree-SHA512: a428e791dfa59c359d3ccc67e8d3a4c1239815d2f6b29898e129700079271c00b3a45f091f70b65a6e54aa00a3d5b678b6da29d2a76b6cd6f946eaa7082ea696

doc/policy/packages.md

@@ -57,3 +57,18 @@ test accepts):
 
    - Warning: Batched fee-bumping may be unsafe for some use cases. Users and application developers
      should take caution if utilizing multi-parent packages.
+
+* Transactions in the package that have the same txid as another transaction already in the mempool
+  will be removed from the package prior to submission ("deduplication").
+
+   - *Rationale*: Node operators are free to set their mempool policies however they please, nodes
+     may receive transactions in different orders, and malicious counterparties may try to take
+     advantage of policy differences to pin or delay propagation of transactions. As such, it's
+     possible for some package transaction(s) to already be in the mempool, and there is no need to
+     repeat validation for those transactions or double-count them in fees.
+
+   - *Rationale*: We want to prevent potential censorship vectors. We should not reject entire
+     packages because we already have one of the transactions. Also, if an attacker first broadcasts
+     a competing package or transaction with a mutated witness, even though the two
+     same-txid-different-witness transactions are conflicting and cannot replace each other, the
+     honest package should still be considered for acceptance.