docs: add some comments to document there types

[RSS1102]

close #219

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ 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

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[RSS1102]

https://github.com/unjs/unbuild/blob/main/src/types.ts#L94-L104

export interface BuildContext {
   ...
  buildEntries: {
    path: string;
    bytes?: number;
    exports?: string[];
    chunks?: string[];
    chunk?: boolean;
    modules?: { id: string; bytes: number }[];
  }[];
  usedImports: Set<string>;
  warnings: Set<string>;
  hooks: Hookable<BuildHooks>;
}

I don't quite understand the significance of these things. Can you give some explanation?