Format BTCMnemonic docs for AppleDoc

[KingParmenides]

Summary

• Convert BTCMnemonic API comments from plain line comments to AppleDoc-compatible documentation blocks.
• Add parameter and return tags for the public initializers.
• Keep this PR scoped to documentation formatting for one class, matching the bounty request in Documentation Format #41.

Verification

• git diff --check
• xcodebuild -list -project CoreBitcoin.xcodeproj could not run here because the active developer directory is Command Line Tools, not full Xcode.

Bounty payout address if accepted: 39Q34P8A7g375yqEr8buvNJkUbRgKfbKQZ