rollup merge of #24665: sw17ch/document-complete-slice-syntax

rollup merge of #24665: sw17ch/document-complete-slice-syntax

The documentation doesn't appear to describe the `&foo[..]` syntax.

I tried looking in `primitive-types.html#slices` and `std/primitive.slice.html`.

There's an example of partially slicing an array in trpl and a mention of `&foo[..]` in [the standard library documentation](https://doc.rust-lang.org/std/primitive.slice.html), but neither place, from what I can see, actually describes the behavior of `&foo[..]`.

+r? @steveklabnik