Don't report missing safety doc for #[target_feature(enable)] attributed functions

[DJMcNab]

changelog: missing_safety_doc: No longer applies for #[target_feature(enable)] attributes functions.

As of Rust 1.86, these functions are safe to call in a context with the specified target features already enabled. I asked Ralf at RustWeek, who thought that these functions probably shouldn't require a safety comment. However, Clippy currently requires this with missing_safety_doc. This PR removes the warning in that case.

---

I also wrote a version which is MSRV aware version, which handled the MSRV change. This is the first commit of this branch. I don't think this is the correct implementation, but I have it for discussion. For this, the changelog would be:

missing_safety_doc: No longer applies for #[target_feature(enable)] attributes functions if your MSRV has target features 1.1.

[rustbot]

Some changes occurred in clippy_lints/src/doc

cc @notriddle

[rustbot]

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