RBF docs mix BIP 125 and Bitcoin Core policy

[noahjoeris]

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: candidate_filter enforces BIP 125 rule 2; was removed from current Core policy.
• examples/synopsis.rs:99-100: links to moving master Core policy and says all rules are followed.
• examples/synopsis.rs:122-123: attributes removed rule 2 to Bitcoin Core policy.
• examples/synopsis.rs:133: says "mempool-replacement policy" without naming the policy/version.
• examples/synopsis.rs:147: 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.