over/underflow vulnerability updated

[0xSandyy]

Related Issue

• Closes: https://github.com/kadenzipfel/smart-contract-vulnerabilities/issues/64

Checklist

• [x] I have read and followed the style guide

Describe the changes you've made:

I updated the overflow and underflow vulnerability by adding more ways in which overflow/underflow can occur after solidity >=0.8.

Type of change

Select the appropriate checkbox:

• [ ] Bug fix (fixing an issue with existing vulnerability data)
• [ ] New feature (adding a new vulnerability or category)
• [x] Documentation update (improving existing information)

Additional Information

I provided just enough information about different concepts like typecasting, inline assembly, etc without going into detail as this report is mostly about exploiting overflow and underflow. But, there are resources in the source section if readers want to learn more.