Fix typescript docs (#4303)

* Add docs on annotations for useState and initial editor value

* Typescript docs: Use initialValue

docs/walkthroughs/01-installing-slate.md

@@ -47,10 +47,10 @@ const App = () => {
 
 Of course we haven't rendered anything, so you won't see any changes.
 
-> If you are using TypeScript, you will also need to extend the `Editor` with `ReactEditor` as per the documentation on [TypeScript](../concepts/12-typescript.md). The example below also includes the custom types required for the rest of this example.
+> If you are using TypeScript, you will also need to extend the `Editor` with `ReactEditor` and add annotations as per the documentation on [TypeScript](../concepts/12-typescript.md). The example below also includes the custom types required for the rest of this example.
 
 ```typescript
-// TypeScript Users only add this code
+// TypeScript users only add this code
 import { BaseEditor } from 'slate'
 import { ReactEditor } from 'slate-react'
 
@@ -65,6 +65,14 @@ declare module 'slate' {
   }
 }
 ```
+```typescript jsx
+// Also you must annotate `useState<Descendant[]>` and the editor's initial value.
+const App = () => {
+  const initialValue : CustomElement = [];
+  const [value, setValue] = useState<Descendant[]>(initialValue)
+  return <Slate value={value} onChange={setValue}>...</Slate>
+}
+```
 
 Next we want to create state for `value`: