Rollup merge of #35613 - matthew-piziak:array-docs-trait-justification, r=steveklabnik
Rollup merge of #35613 - matthew-piziak:array-docs-trait-justification, r=steveklabnik

provide additional justification for array interface design

Explain why Rust does not implement traits for large arrays.

Explain why most methods are implemented on slices rather than arrays.

Note: I'm dipping my toes in the water with a tiny PR. Especially looking for feedback on wording and style. Points of concern: appropriate level of top-level explanation; foreshadowing (is it appropriate to imply that we expect Rust's type system to eventually support size-generic arrays?); using `Foo` and `Bar` as type variables instead of e.g. `T` and `S`.

@peschkaj

graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust