Add some clarifying documentation around Editor schema-specific methods

docs/api/editor.md

@@ -316,6 +316,20 @@ Call a function, deferring normalization until after it completes.
 
 ### Schema-specific methods to override
 
+These methods can and should be replaced to modify the original behavior of the editor which is a part of building [Plugins](/concepts/07-plugins). When you modify behavior, call the original method where appropriate. For example, a plugin that marks image nodes as "void":
+
+```js
+const withImages = editor => {
+  const { isVoid } = editor
+
+  editor.isVoid = element => {
+    return element.type === 'image' ? true : isVoid(element)
+  }
+
+  return editor
+}
+```
+
 ###### `isInline(element: Element)`
 
 Check if a value is an inline `Element` object.