Clarify docs for split_at_mut
Clarify docs for split_at_mut

The `&mut` here didn't make immediate sense to me. Keep the docs for this function consistent with the non-mut version.
graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust