Rollup merge of #111065 - est31:send_mut_ref, r=m-ou-se

Explicitly document how Send and Sync relate to references

Some of these relations were already mentioned in the text, but that Send is implemented for &mut impl Send was not mentioned, neither did the docs list when &T is Sync. Inspired by the discussion in #110961.

[Proof](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=ed77bfc3c77ba664400ebc2734f500e6) based on `@lukas-code` 's [example](rust-lang/rust#110961 (comment)).

Author: matthiaskrgr