Documents how to implement a Void Element. (#4924)

* Documents how to implement a Void Element.

* Tightens up phrasing

docs/api/nodes/element.md

@@ -11,6 +11,7 @@ interface Element {
 - [Behavior Types](element.md#element-behavior-types)
   - [Block vs. Inline](element.md#block-vs-inline)
   - [Void vs Not Void](element.md#void-vs-not-void)
+    - [Rendering Void Elements](element.md#rendering-void-elements)
 - [Static methods](element.md#static-methods)
   - [Retrieval methods](element.md#retrieval-methods)
   - [Check methods](element.md#check-methods)
@@ -30,6 +31,25 @@ A "block" element can only be siblings with other "block" elements. An "inline"
 
 In a not "void" element, Slate handles the rendering of its `children` (e.g. in a paragraph where the `Text` and `Inline` children are rendered by Slate). In a "void" element, the `children` are rendered by the `Element`'s render code.
 
+#### Rendering Void Elements
+
+Void Elements must
+
+- always have one empty child text node (for selection)
+- render using `attributes` and `children` (so, their outermost HTML element **can't** be an HTML void element)
+- set `contentEditable={false}` (for Firefox)
+
+Typical rendering code will resemble this `thematic-break` (horizontal rule) element:
+
+```javascript
+return (
+  <div {...attributes} contentEditable={false}>
+    {children}
+    <hr />
+  </div>
+)
+```
+
 ## Static methods
 
 ### Retrieval methods

docs/concepts/02-nodes.md

@@ -116,7 +116,7 @@ Elements default to being non-void, meaning that their children are fully editab
 
 > 🤖 This is a concept borrowed from the HTML spec, see [Void Elements](https://www.w3.org/TR/2011/WD-html-markup-20110405/syntax.html#void-element).
 
-You can define which elements are treated as void by overriding the `editor.isVoid` function. \(By default it always returns `false`.\)
+You can define which elements are treated as void by overriding the `editor.isVoid` function. \(By default it always returns `false`.\) See [Rendering Void Elements](../api/nodes/element.md#rendering-void-elements) for implementation details.
 
 ## `Text`
 
@@ -138,4 +138,4 @@ const text = {
 ```
 
 Text nodes too can contain any custom properties you want, and that's how you implement custom formatting like **bold**, _italic_, `code`, etc.
-These custom properties are sometimes called [marks](../api/nodes/editor#mark-methods).
+These custom properties are sometimes called [marks](../api/nodes/editor.md#mark-methods).