Rollup merge of #44325 - nielsegberts:master, r=steveklabnik
Rollup merge of #44325 - nielsegberts:master, r=steveklabnik

Make slice::split_at_mut example demonstrate mutability

Moved the examples from split_at_mut to split_at so the example at
split_at_mut can just demonstrate mutability.

See #44314

r? @steveklabnik

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