Add missing documentation for rule.marks (#1717)

docs/reference/slate/schema.md

@@ -73,6 +73,7 @@ A dictionary of inlines by type, each with its own set of validation rules.
   isVoid: Boolean,
   last: Object,
   nodes: Array,
+  marks: Array,
   normalize: Function,
   parent: Object,
   text: RegExp,
@@ -148,6 +149,18 @@ Will validate a node's children. The `nodes` definitions can declare the `object
 
 > 🤖 The `nodes` array is order-sensitive! The example above will require that the first node be either an `image` or `video`, and that it be followed by one or more `paragraph` nodes.
 
+### `marks`
+
+`Array`
+
+```js
+{
+  marks: ['italic']
+}
+```
+
+Will validate a node's marks. The `marks` definitions can declare a list of marks type to be allowed. If declared, any marks that are not in the list will be removed.
+
 ### `normalize`
 
 `normalize(change: Change, violation: String, context: Object) => Void`