Rollup merge of #40505 - frewsxcv:hash-docs, r=alexcrichton

A few improvements to the `core::hash` top-level docs.

Primarily opened to address the concerns brought up in
https://github.com/rust-lang/rust/issues/40498.

* run rustfmt on code blocks
* use `DefaultHasher` instead of deprecated `SipHasher`
* rename `hash` to `calculate_hash` to prevent confusion with the `hash`
  method

diff --cc src/libcore/hash/mod.rs
Simple merge