Rollup merge of #142262 - aDotInTheVoid:nomemchr, r=Noratrieb

Mark `core::slice::memchr` as `#[doc(hidden)]`

It's purely internal, and not intended to be a public API, even on nightly. This stops it showing up and being misleading in rustdoc search.

It also mirrors the (also internal) `core::slice::sort` module.

Author: fmease