Rollup merge of rust-lang#104111 - yancyribbens:add-mutable-to-the-de…

…scription-for-as-simd-mut, r=scottmcm

rustdoc: Add mutable to the description

Add mutable the description to differentiate [as_simd](https://github.com/rust-lang/rust/blob/master/library/core/src/slice/mod.rs#L3654) from [as_simd_mut](https://github.com/rust-lang/rust/blob/master/library/core/src/slice/mod.rs#L3654).