message: [doc] clarify RBF difference from BIP125
  It can be argued that BIP125#1 signaling is honored because
  descendants of signaling transactions are replaceable by RBF.
  Regardless, since there are multiple details in our RBF policy that are
  not captured in BIP125, point to our doc instead.
diff: @@ -32,7 +32,7 @@ BIPs that are implemented by Bitcoin Core (up-to-date up to **v22.0**):
  * [`BIP 111`](https://github.com/bitcoin/bips/blob/master/bip-0111.mediawiki): `NODE_BLOOM` service bit added, and enforced for all peer versions as of **v0.13.0** ([PR #6579](https://github.com/bitcoin/bitcoin/pull/6579) and [PR #6641](https://github.com/bitcoin/bitcoin/pull/6641)).
  * [`BIP 112`](https://github.com/bitcoin/bips/blob/master/bip-0112.mediawiki): The CHECKSEQUENCEVERIFY opcode has been implemented since **v0.12.1** ([PR #7524](https://github.com/bitcoin/bitcoin/pull/7524)), and has been *buried* since **v0.19.0** ([PR #16060](https://github.com/bitcoin/bitcoin/pull/16060)).
  * [`BIP 113`](https://github.com/bitcoin/bips/blob/master/bip-0113.mediawiki): Median time past lock-time calculations have been implemented since **v0.12.1** ([PR #6566](https://github.com/bitcoin/bitcoin/pull/6566)), and has been *buried* since **v0.19.0** ([PR #16060](https://github.com/bitcoin/bitcoin/pull/16060)).
  -* [`BIP 125`](https://github.com/bitcoin/bips/blob/master/bip-0125.mediawiki): Opt-in full replace-by-fee signaling honoured in mempool and mining as of **v0.12.0** ([PR 6871](https://github.com/bitcoin/bitcoin/pull/6871)). Enabled by default in the wallet GUI as of **v0.18.1** ([PR #11605](https://github.com/bitcoin/bitcoin/pull/11605))
  +* [`BIP 125`](https://github.com/bitcoin/bips/blob/master/bip-0125.mediawiki): Opt-in full replace-by-fee signaling partially implemented. See doc/policy/mempool-replacements.md.
  * [`BIP 130`](https://github.com/bitcoin/bips/blob/master/bip-0130.mediawiki): direct headers announcement is negotiated with peer versions `>=70012` as of **v0.12.0** ([PR 6494](https://github.com/bitcoin/bitcoin/pull/6494)).
  * [`BIP 133`](https://github.com/bitcoin/bips/blob/master/bip-0133.mediawiki): feefilter messages are respected and sent for peer versions `>=70013` as of **v0.13.0** ([PR 7542](https://github.com/bitcoin/bitcoin/pull/7542)).
  * [`BIP 141`](https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki): Segregated Witness (Consensus Layer) as of **v0.13.0** ([PR 8149](https://github.com/bitcoin/bitcoin/pull/8149)), defined for mainnet as of **v0.13.1** ([PR 8937](https://github.com/bitcoin/bitcoin/pull/8937)), and *buried* since **v0.19.0** ([PR #16060](https://github.com/bitcoin/bitcoin/pull/16060)).
  