Document all unsafe blocks #5487
Description
Starting with #5484 we're adding the clippy::undocumented_unsafe_blocks lint to instance.rs. We should expand this across the codebase.
For now this probably requires picking a file, enabling the lint at the top, and fixing the undocumented blocks.
Once we've covered enough of the codebase we should be able to switch to having a top-level warn and per-file allows. We can then work to remove the per-file allows.