Rollup merge of #120449 - udoprog:document-unsized-rc-arc-from-raw, r=m-ou-se

Document requirements for unsized {Rc,Arc}::from_raw

This seems to be implied due to these types supporting operation-less unsized coercions. Taken together with the [established behavior of a wide to thin pointer cast](rust-lang/reference#1451) it would enable unsafe downcasting of these containers.

Note that the term "data pointer" is adopted from rust-lang/rfcs#3559

See also this [internals thread](https://internals.rust-lang.org/t/can-unsafe-smart-pointer-downcasts-be-correct/20229/2).

Author: GuillaumeGomez