docs: enhance module docstring with clear purpose statement

Add clarification that this module provides a high-performance alternative
to pydantic-settings, making the value proposition more explicit.

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

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

Author: vilsonrodrigues

src/msgspec_ext/settings.py

@@ -1,4 +1,7 @@
-"""Optimized settings management using msgspec.Struct and bulk JSON decoding."""
+"""Optimized settings management using msgspec.Struct and bulk JSON decoding.
+
+This module provides a high-performance alternative to pydantic-settings.
+"""
 
 import os
 from pathlib import Path