docs: clarify AsRef and AsMut exercise

[fuentez145]

Summary

• Clarify how AsRef and AsMut let functions accept different input types.
• Explain the AsRef<str> and AsMut<u32> bounds in conversions5.
• Add a short practical explanation to the conversions README.
• Keep the exercise comments synchronized with the solution.

Test plan

• cargo fmt --all -- --check
• cargo clippy -- --deny warnings
• cargo test --workspace
• cargo dev check --require-solutions
• git diff --check

Closes #2199

[senekor]

Please clarify your use of LLMs when creating this PR.

[fuentez145]

I used GPT Luna to help me investigate issue #2199 and draft the documentation changes. I reviewed the suggestions myself, made the final edits, and verified the changes with the local Rustlings checks. All GitHub Actions checks also passed.

[mo8it]

I prefer documentation to be written by humans. I don't like the explanation provided.