Update datasets.py comments

[glenn-jocher]

Improved comments for dataloaders.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced data loader docstrings for improved clarity and functionality explanation.

📊 Key Changes

• Updated comments to provide clear explanations of the data loaders' purposes right within their class definitions.
• LoadImages, LoadWebcam, and LoadStreams classes have added explanatory comments describing their use cases.
• LoadImagesAndLabels class now includes a comment indicating its use in training and validation.
• Enhanced comments for load_mosaic and load_mosaic9 methods to clarify that they load images into 4-image and 9-image mosaics, respectively.

🎯 Purpose & Impact

• 📝 Better Documentation: The new comments serve to guide users more clearly on how to use the different data loaders for images, videos, webcams, streams, and during training/validation.
• 💡 Improved Understanding: With these comments, users, especially new ones, can better understand the purpose of each class and function at a glance, potentially speeding up their development process.
• ✨ Enhanced Usability: As users get more information directly from the source code, these changes can improve the usability of the YOLOv5 library, making it more user-friendly.