Rollup merge of #106889 - scottmcm:windows-mut, r=cuviper

Mention the lack of `windows_mut` in `windows`

This is a common request, going back to at least 2015 (#23783), so mention in the docs that it can't be done and offer a workaround using <https://doc.rust-lang.org/std/cell/struct.Cell.html#method.as_slice_of_cells>.

(See also URLO threads like <https://internals.rust-lang.org/t/a-windows-mut-method-on-slice/16941/10?u=scottmcm>.)

diff --cc library/core/src/slice/mod.rs
Simple merge