fix(criterion_compat): let benches denying missing_docs compile

[LucaCappelletti94]

Bench targets that deny missing_docs compile against criterion 0.8 but fail against codspeed-criterion-compat.

Criterion fixed its macro in 0.8.0 (criterion-rs/criterion.rs#26) by putting a doc attribute on the generated function. This PR applies the same attribute to both criterion_group! expansions and makes the instrumented main private, which is what the fork and upstream criterion already emit.

When this is fixed, we may be able to start using this in apache/datafusion-sqlparser-rs#2587

[greptile-apps]

Confidence Score: 5/5

[Low risk] Adds a benchmark test and fixes macro documentation.

The PR appears safe to merge.

Summary

The PR documents the public functions generated by both Criterion group macros, makes the instrumented entry point private, and adds a missing_docs benchmark to CI. No actionable issue was identified.

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