Rollup merge of #120110 - invpt:patch-1, r=the8472

Update documentation for Vec::into_boxed_slice to be more clear about excess capacity

Currently, the documentation for Vec::into_boxed_slice says that "if the vector has excess capacity, its items will be moved into a newly-allocated buffer with exactly the right capacity." This is misleading, as copies do not necessarily occur, depending on if the allocator supports in-place shrinking. I copied some of the wording from shrink_to_fit, though it could potentially still be worded better than this.

Author: matthiaskrgr