docs: Added missing piece information on using withCount()

[justinsamuel92]

🔗 Linked issue

Minor Enhancement in the documentation which can actually save a lot of time for developers while using withCount()

❓ Type of change

• 🐞 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)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

An important update for getting count in the Model while using withCount(). Surely will save some time for developers

📝 Checklist

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