docs(kms): correct the stale backend_info and rotation error claims

Two claims in docs/operations/kms-backend-security.md described code that
later changes removed or replaced.

The Vault KV2 section promised a `backend_info` metadata report of
`at_rest_protection: vault-kv2-acl`. That implementation and its assertion
test were removed by #5501 (76b3c085b5); the KMS crate has no BackendInfo
type at all today. The surviving field-level expression of the boundary is
the backup contract, where VaultKv2 maps to AtRestProtection::StorageOnly
and is serialized into a bundle manifest's at_rest_protection field as
`storage-only`. Say that instead, and say plainly that nothing reports the
boundary at runtime.

The rotation section named `InvalidOperation` as the error Local and Static
return. Neither backend advertises `rotate` in capabilities(), so rotate_key
falls through to the default trait impl and returns UnsupportedCapability,
which the contract tests already pin. InvalidOperation survives only in the
cfg(test) LocalKmsClient::rotate_key, off the product path. The "rotation is
rejected" conclusion was correct; only the error type was wrong.

Also drop the same dead vault-kv2-acl claim from the AtRestProtection
doc comment so the two do not drift apart again.

Documentation only; no behavior change.

crates/kms/src/backup/capability.rs

@@ -65,8 +65,7 @@ pub enum AtRestProtection {
     /// for bundling purposes: re-wrap is mandatory.
     LegacyUnspecified,
     /// Vault KV2 as currently shipped: material confidentiality relies on
-    /// Vault ACLs, KV2 at-rest encryption, and TLS only (the backend reports
-    /// `at_rest_protection = "vault-kv2-acl"`); RustFS applies no
+    /// Vault ACLs, KV2 at-rest encryption, and TLS only; RustFS applies no
     /// cryptographic wrapping of its own.
     StorageOnly,
     /// Vault KV2 with material wrapped by Vault Transit before storage. Not

docs/operations/kms-backend-security.md

@@ -21,7 +21,7 @@ The Vault KV2 backend uses Vault purely as a **secure storage** service:
 - Master key material is generated by RustFS and written to KV v2 as a Base64-encoded value (`encrypted_key_material` is an encoding, not a ciphertext).
 - The backend never calls the Vault Transit engine. The `mount_path` configuration field and the `RUSTFS_KMS_VAULT_MOUNT_PATH` environment variable are deprecated leftovers: they are accepted for compatibility and ignored.
 - Data-encryption keys (DEKs) handed to the object-encryption path are still wrapped with AES-256-GCM under the master key; the statement above concerns the master key's storage in Vault, not the DEK envelope.
-- The backend reports this boundary in its `backend_info` metadata as `at_rest_protection: vault-kv2-acl`.
+- The backend exposes no runtime report of this boundary. It is expressed as a field in exactly one place: a backup bundle manifest records `at_rest_protection: storage-only` for this backend, meaning RustFS adds no cryptographic wrapping of its own on top of Vault's storage-level protection.
 - Key rotation retains every historical master key version as an immutable record under `{prefix}/{key_id}/versions/{N}` and only then moves the current-version pointer; see [Master key rotation](#master-key-rotation-retention-destruction-and-upgrade-ordering) for the retention preconditions and the cluster-upgrade ordering constraint.
 
 > **Warning: KV read access is equivalent to holding the master keys.**
@@ -51,7 +51,7 @@ Notes:
 
 ## Master key rotation: retention, destruction, and upgrade ordering
 
-Rotation support differs per backend. Local and Static reject rotation outright (`InvalidOperation`); their single key material is never overwritten. Vault Transit delegates rotation to the Transit engine's own key versioning (ciphertext is version-prefixed, e.g. `vault:v1:...`). Vault KV2 rotates by retaining every historical version, as described below. Rotation is reachable through the admin API as `POST /rustfs/admin/v3/kms/keys/rotate`, which the route policy classifies as high risk and gates behind `kms:RotateKey`; it is not exposed through the S3 surface. The upgrade ordering constraint below therefore applies to an operator action, not only to a call from inside the process.
+Rotation support differs per backend. Local and Static reject rotation outright: neither advertises the `rotate` capability, so the call fails with `UnsupportedCapability` before reaching any backend logic, and their single key material is never overwritten. Vault Transit delegates rotation to the Transit engine's own key versioning (ciphertext is version-prefixed, e.g. `vault:v1:...`). Vault KV2 rotates by retaining every historical version, as described below. Rotation is reachable through the admin API as `POST /rustfs/admin/v3/kms/keys/rotate`, which the route policy classifies as high risk and gates behind `kms:RotateKey`; it is not exposed through the S3 surface. The upgrade ordering constraint below therefore applies to an operator action, not only to a call from inside the process.
 
 ### Vault KV2 versioned retention model