docs: enhance benchmark documentation with value construction convention

[TKorr]

• Added a section on value construction convention to the benchmark files, clarifying how benchmark entry points utilize Fn(u64) -> Arc<V> for value storage.
• Emphasized the importance of using the cheapest possible payload to focus on cache policy behavior rather than allocator costs.

These updates improve the clarity of benchmark usage and ensure users understand the implications of value construction on performance metrics.

Description

Related Issue

Fixes #

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 📝 Documentation update
• 🔧 Refactoring (no functional changes)
• ⚡ Performance improvement
• 🧪 Test addition or modification

How Has This Been Tested?

• Unit tests
• Integration tests
• Manual testing

Test environment:

• OS:
• Rust version:

Checklist

• My code follows the project's coding standards
• I have run cargo fmt and cargo clippy
• I have added tests for my changes
• All new and existing tests pass (cargo test)
• I have updated the documentation as needed
• I have added an entry to CHANGELOG.md (if applicable)

Screenshots (if applicable)

Additional Notes