don't allow undocumented unsafe blocks

We already have this as a requirement for merging PRs as part of the
PR checklist template, but it's better to enforce it. This comes with
the disadvantage that we will need to update all comments and add the
prefix "SAFETY: " as required by the clippy check.

Signed-off-by: Andreea Florescu <fandree@amazon.com>

.buildkite/test_description.json

@@ -38,7 +38,7 @@
     },
     {
       "test_name": "clippy",
-      "command": "cargo clippy --workspace --bins --examples --benches --all-features --all-targets -- -D warnings",
+      "command": "cargo clippy --workspace --bins --examples --benches --all-features --all-targets -- -D warnings -D clippy::undocumented_unsafe_blocks",
       "platform": [
         "x86_64",
         "aarch64"