Rollup merge of #65877 - lzutao:iter-chain-once, r=Centril
Rollup merge of #65877 - lzutao:iter-chain-once, r=Centril

doc: introduce `once` in `iter::chain` document

I find it hard to find which one to use with `chain` when I only need to
chain one value. Also [`once`][1] talks about `chain`.

[1]: https://doc.rust-lang.org/nightly/std/iter/fn.once.html

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