doc: trd: store perms: update impl type

Needs a wrapper struct to enforce capabilities in the constructor.

doc/reference/trd-storage-permissions.md

@@ -248,7 +248,9 @@ The `StoragePermissions` type is capable of holding storage permissions in
 different formats. In general, the type looks like:
 
 ```rust
-pub enum StoragePermissions {
+pub struct StoragePermissions(StoragePermissionsPrivate);
+
+enum StoragePermissionsPrivate {
     SelfOnly(core::num::NonZeroU32),
     FixedSize(FixedSizePermissions),
     Listed(ListedPermissions),
@@ -261,8 +263,10 @@ Each variant is a different method for representing and storing storage
 permissions. For example, `FixedSize` contains fixed size lists of permissions,
 where as `Null` grants no storage permissions.
 
-The `StoragePermissions` type includes multiple constructors for instantiating
-storage permissions.
+The `StoragePermissions` struct includes multiple constructors for instantiating
+storage permissions. The struct wraps the enum to ensure that permissions can
+only be created with those constructors. The constructors require a capability
+to use so only trusted code can create storage permissions.
 
 
 7 Specifying Permissions