Auto merge of rust-lang#148526 - reddevilmidzy:docs, r=Mark-Simulacrum

Expand pow docs with special-case tests

resolve: rust-lang#148316

Files changed:

* library/std/src/num: f32.rs, f64.rs,
  * powi
  * powf
* library/std/src/num: f16.rs, f128.rs
  * powf
* library/core/src/num: f16.rs, f128.rs
  * powi
* library/core/src/num: int_macros.rs, uint_macros.rs
  * checked_pow
  * strict_pow
  * saturating_pow
  * wrapping_pow
  * overflowing_pow
  * pow

Author: bors