Rollup merge of #124692 - workingjubilee:document-no-double-pointer-coercion-happens, r=compiler-errors

We do not coerce `&mut &mut T -> *mut mut T`

Resolves #34117 by declaring it to be "working as intended" until someone RFCs it or whatever other lang proposal would be required. It seems a bit of a footgun, but perhaps there are strong reasons to allow it anyways. Seeing as how I often have to be mindful to not allow a pointer to coerce the wrong way in my FFI work, I am inclined to think not, but perhaps it's fine in some use-case and that's actually more common?

Author: matthiaskrgr