RBF docs mix BIP 125 and Bitcoin Core policy

The RBF docs/comments currently mix BIP 125 rule numbers and Bitcoin Core's `mempool-replacements.md` wording as if they were the same spec.

Bitcoin Core's replacement policy has diverged from BIP 125 over time:

- **Rule 1:** Removed in Core policy: full RBF default in v28.0; `-mempoolfullrbf` removed in v29.0
- **Rule 2:** Removed in Core policy in v31.0.
- **Rule 3:** Unchanged.
- **Rule 4:** Changed in Core policy to use `-incrementalrelayfee`
- **Rule 5:** Changed in Core policy in v31.0.
- **Rule 6:** Core-only rule; changed in Core policy in v31.0.

This makes several comments ambiguous or factually wrong depending on which policy/version the reader assumes.

### Examples
- [`src/rbf.rs:81-93`](https://github.com/bitcoindevkit/bdk-tx/blob/8142a660a6ce210860f63da39ccd483b6b47edf5/src/rbf.rs#L81-L84): `candidate_filter` enforces BIP 125 rule 2; was removed from current Core policy.
- [`examples/synopsis.rs:99-100`](https://github.com/bitcoindevkit/bdk-tx/blob/8142a660a6ce210860f63da39ccd483b6b47edf5/examples/synopsis.rs#L99-L100): links to moving `master` Core policy and says all rules are followed.
- [`examples/synopsis.rs:122-123`](https://github.com/bitcoindevkit/bdk-tx/blob/8142a660a6ce210860f63da39ccd483b6b47edf5/examples/synopsis.rs#L122-L123): attributes removed rule 2 to Bitcoin Core policy.
- [`examples/synopsis.rs:133`](https://github.com/bitcoindevkit/bdk-tx/blob/8142a660a6ce210860f63da39ccd483b6b47edf5/examples/synopsis.rs#L133): says "mempool-replacement policy" without naming the policy/version.
- [`examples/synopsis.rs:147`](https://github.com/bitcoindevkit/bdk-tx/blob/8142a660a6ce210860f63da39ccd483b6b47edf5/examples/synopsis.rs#L147): says "rules 4 and 6" without naming Core/version.

### Possible fix
- Follow a specific Bitcoin Core policy version and link to tagged Core docs instead of `master`
- Document any behavior that intentionally differs from that pinned Core policy, e.g. stricter BIP 125 checks.