Add mention of extended IProductSnapshot interfaces

[jemayn]

📋 Description

I've recently spent way more time than I'd like to admit on figuring out how to add a weight onto a product in Umbraco Commerce when fetching products through a Product Adapter.
This PR adds a note on the other extended IProductSnapshot interfaces that add additional data to your product that may be useful.

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• Code blocks are correctly formatted.
• Sentences are short and clear (preferably under 25 words).
• Passive voice and first-person language (“we”, “I”) are avoided.
• Any code examples or instructions have been tested.

Product & Version (if relevant)

I've been working on Umbraco Commerce 16.4.1, but I'd assume it to be the same for all other versions.

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide