Rollup merge of #99570 - XrXr:box-from-slice-docs, r=thomcc

Box::from(slice): Clarify that contents are copied

A colleague mentioned that they interpreted the old text
as saying that only the pointer and the length are copied.
Add a clause so it is more clear that the pointed to contents
are also copied.

diff --cc library/alloc/src/boxed.rs
Simple merge