Remove answer-revealing exercise hints

[mre]

Summary

• remove the complete get_env_var expression from the learner-facing doc comment and prose
• keep the required behavior and relevant standard-library APIs without composing the answer
• remove the complete write! call from display_temperature's doc comment
• mirror documentation cleanup in solution files so revealed/reference code stays consistent

Validation

• cargo fmt --all --check
• cargo clippy --locked --lib --bins -- -D warnings
• cargo test --locked --lib --bins
• ./scripts/check-examples.sh
• REQUIRE_COMPLETE=1 ./scripts/check-solutions.sh (75/75)