Auto merge of #7321 - flip1995:doc-release-stable-branch, r=xFrednet
Auto merge of #7321 - flip1995:doc-release-stable-branch, r=xFrednet

Release doc: add explanation for updating the stable branch

r? `@xFrednet`

[Rendered](https://github.com/flip1995/rust-clippy/blob/doc-release-stable-branch/doc/release.md)

changelog: none

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