Allow control of Slate's event handler execution in custom event handler API (#4299)

Co-authored-by: Georg Berecz <gberecz@palantir.com>
Co-authored-by: Claudéric Demers <clauderic.d@gmail.com>

docs/libraries/slate-react.md

@@ -18,6 +18,52 @@ React components for rendering Slate editors
 
 The main Slate editor.
 
+#### Event Handling
+
+By default, the `Editable` component comes with a set of event handlers that handle typical rich-text editing behaviors (for example, it implements its own `onCopy`, `onPaste`, `onDrop`, and `onKeyDown` handlers).
+
+In some cases you may want to extend or override Slate's default behavior, which can be done by passing your own event handler(s) to the `Editable` component.
+
+Your custom event handler can control whether or not Slate should execute its own event handling for a given event after your handler runs depending on the return value of your event handler as described below.
+
+```jsx
+import {Editable} from 'slate-react';
+
+function MyEditor() {
+  const onClick = event => {
+    // Implement custom event logic...
+
+    // When no value is returned, Slate will execute its own event handler when
+    // neither isDefaultPrevented nor isPropagationStopped was set on the event
+  };
+
+  const onDrop = event => {
+    // Implement custom event logic...
+
+    // No matter the state of the event, treat it as being handled by returning
+    // true here, Slate will skip its own event handler
+    return true;
+  };
+
+  const onDragStart = event => {
+    // Implement custom event logic...
+
+    // No matter the status of the event, treat event as *not* being handled by
+    // returning false, Slate will exectue its own event handler afterward
+    return false;
+  };
+
+  return (
+    <Editable
+      onClick={onClick}
+      onDrop={onDrop}
+      onDragStart={onDragStart}
+      {/*...*/}
+    />
+  )
+}
+```
+
 ### `DefaultElement(props: RenderElementProps)`
 
 The default element renderer.