Rollup merge of #151612 - tgross35:cold-path-doc, r=scottmcm

Update documentation for `cold_path`, `likely`, and `unlikely`

* Add a note recommending benchmarks to `cold_path`, as other hints have
* Note that `cold_path` can be used to implement `likely` and `unlikely`
* Update the tracking issue for the `likely_unlikely` feature

Tracking issue: rust-lang/rust#136873
Tracking issue: rust-lang/rust#151619

Author: Zalathar