Auto merge of #40634 - GuillaumeGomez:refcell-docs, r=steveklabnik
Auto merge of #40634 - GuillaumeGomez:refcell-docs, r=steveklabnik

Add more explanation on RefCell::get_mut

Fixes #40203.

r? @rust-lang/docs

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