Add missing allowed_managed_key param to mount creation doc (hashicor…

…p#23948)

- We had updated the mount tune api, but missed it within the
   creation api docs

website/content/api-docs/system/mounts.mdx

@@ -171,6 +171,9 @@ This endpoint enables a new secrets engine at the given path.
     unversioned plugin that may have been registered, the latest versioned plugin
     registered, or a built-in plugin in that order of precendence.
 
+  - `allowed_managed_keys` `(array: [])` - List of managed key registry entry names
+    that the mount in question is allowed to access.
+
 - `options` `(map<string|string>: nil)` - Specifies mount type specific options
   that are passed to the backend.