Merge bitcoin/bitcoin#25382: doc requirement that replacement must have higher feerate than direct conflicts

2224bcabc4 [doc] RBF feerate rule (glozow)

Pull request description:

  RBF policy requires the replacement transaction have a higher feerate than each of the directly conflicting transactions (see `PaysMoreThanConflicts`).
  It was pointed out that this rule is undocumented: https://github.com/bitcoin/bitcoin/pull/25038#discussion_r889064935

ACKs for top commit:
  laanwj:
    ACK 2224bcabc4
  w0xlt:
    ACK 2224bcabc4
  darosior:
    ACK 2224bcabc4
  ariard:
    ACK 2224bcab
  t-bast:
    ACK 2224bcabc4

Tree-SHA512: 0d3915100973b66d115c3294f3037d0c5473c00236c8823a4b2fe12ff172457af56c295b41ac0ef983de030f40f0817c046bb486bf60a5a593d1c4524fe1b9d2

doc/policy/mempool-replacements.md

@@ -51,6 +51,13 @@ other consensus and policy rules, each of the following conditions are met:
    significant portions of the node's mempool using replacements with multiple directly conflicting
    transactions, each with large descendant sets.
 
+6. The replacement transaction's feerate is greater than the feerates of all directly conflicting
+   transactions.
+
+   *Rationale*: This rule was originally intended to ensure that the replacement transaction is
+   preferable for block-inclusion, compared to what would be removed from the mempool. This rule
+   predates ancestor feerate-based transaction selection.
+
 This set of rules is similar but distinct from BIP125.
 
 ## History