Format BTCKey docs for AppleDoc

[KingParmenides]

Summary

• Convert BTCKey API comments from plain line comments to AppleDoc-compatible documentation blocks.
• Add parameter, return, discussion, see, and section tags for key initialization, signing, address import/export, compact signatures, message signing, and canonical encoding checks.
• 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