docs: improve typescript docs, fix #2407

docs/guide/typescript.md

@@ -45,6 +45,8 @@ export interface CustomExtensionStorage {
 }
 
 const CustomExtension = Extension.create<{}, CustomExtensionStorage>({
+  name: 'customExtension',
+
   addStorage() {
     return {
       awesomeness: 100,
@@ -53,6 +55,14 @@ const CustomExtension = Extension.create<{}, CustomExtensionStorage>({
 })
 ```
 
+When using storage outside of the extension you have to manually set the type.
+
+```
+import { CustomExtensionStorage } from './custom-extension
+
+const customStorage = editor.storage.customExtension as CustomExtensionStorage
+```
+
 ### Command type
 The core package also exports a `Command` type, which needs to be added to all commands that you specify in your code. Here is an example: