docs(kms): correct the KV2 at-rest boundary and rotation rejection claims
The Vault KV2 section claimed the backend reports its confidentiality
boundary as `at_rest_protection: vault-kv2-acl` through `backend_info`.
That accessor, the `BackendInfo` type and its assertion test were removed
in rustfs/rustfs#5501, and no replacement reports the boundary. State
instead that the boundary is documented only, that `kms/status` exposes a
capability matrix covering supported operations rather than key-material
location, and that the backup manifest's `at_rest_protection` field is a
bundle declaration rather than a backend self-report.
The rotation section named `InvalidOperation` as the error Local and
Static return. Neither advertises `rotate`, so the product path falls to
the shared `KmsBackend` default and returns `UnsupportedCapability`;
`InvalidOperation` survives only in a test-only client helper.
Zhengchao An
8bb147cb70