Fixed two types of documentation warnings:
1. Removed reference to private function Protocol.impl_for/1 in CHANGELOG.md
   - Changed to "protocol introspection" description
2. Fixed LICENSE file references in README.md
   - Changed relative links to absolute GitHub URLs
   - Prevents ExDoc from looking for LICENSE in doc output directory

All mix docs warnings now resolved.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>