[doc] remove non-signaling mentions of BIP125

Our RBF policy is different from the rules specified in BIP125. For
example, the BIP does not mention Rule 6, and our Rule 4 uses the
(configurable) incremental relay feerate (distinct from the
minimum relay feerate). Those interested in our policy should refer to
doc/policy/mempool-replacements.md instead. These rules may also
continue to diverge with package RBF and other RBF improvements. Keep
references to the BIP125 signaling wrt sequence numbers, since that is
still correct and widely used. It is helpful to refer to this as "BIP125
signaling" since it is unambiguous and succint, especially if we have
multiple ways to signal replaceability in the future.

The rule numbers in doc/policy/mempool-replacements.md correspond
largely to those of BIP 125, so we can still refer to them like "Rule 5."

src/wallet/feebumper.cpp

@@ -128,8 +128,8 @@ static CFeeRate EstimateFeeRate(const CWallet& wallet, const CWalletTx& wtx, con
     // WALLET_INCREMENTAL_RELAY_FEE value to future proof against changes to
     // network wide policy for incremental relay fee that our node may not be
     // aware of. This ensures we're over the required relay fee rate
-    // (BIP 125 rule 4).  The replacement tx will be at least as large as the
-    // original tx, so the total fee will be greater (BIP 125 rule 3)
+    // (Rule 4).  The replacement tx will be at least as large as the
+    // original tx, so the total fee will be greater (Rule 3)
     CFeeRate node_incremental_relay_fee = wallet.chain().relayIncrementalFee();
     CFeeRate wallet_incremental_relay_fee = CFeeRate(WALLET_INCREMENTAL_RELAY_FEE);
     feerate += std::max(node_incremental_relay_fee, wallet_incremental_relay_fee);