Rollup merge of #41536 - steveklabnik:arc-and-send, r=burntsushi
Rollup merge of #41536 - steveklabnik:arc-and-send, r=burntsushi

Improve docs on Arc<T> and Send/Sync

This is something I always forget, so let's actually
explain in the docs.

I didn't fully link up everything here, but I'd like to make sure that the wording is okay before I bother.

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