docs: clarify async behavior and default script attributes

[nexoscreator]

🔗 Linked issue

Resolves #304 : Some users expected async to be applied by default, leading to confusion. This update makes the behavior explicit.

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

• Updated documentation to clarify that async is not applied by default because defer is used.
• Added a note explaining how to explicitly enable async by disabling defer.
• Improved readability of default script attributes section.

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[harlan-zw]

Thanks, this is an improvement but there's still some issues, like fetchpriority is conditional added based on the trigger but let's go with this for now.