docs(api): document addMissingDependency for loader context (#6132)

* document addMissingDependency

* Apply suggestions from code review

Co-authored-by: Nitin Kumar <snitin315@gmail.com>

Co-authored-by: Sam Chen <chenxsan@gmail.com>
Co-authored-by: Nitin Kumar <snitin315@gmail.com>

Author: 3 people

src/content/api/loaders.mdx

@@ -210,7 +210,15 @@ addDependency(file: string)
 dependency(file: string) // shortcut
 ```
 
-Add a file as dependency of the loader result in order to make them watchable. For example, [`sass-loader`](https://github.com/webpack-contrib/sass-loader), [`less-loader`](https://github.com/webpack-contrib/less-loader) uses this to recompile whenever any imported `css` file changes.
+Add an existing file as a dependency of the loader result in order to make them watchable. For example, [`sass-loader`](https://github.com/webpack-contrib/sass-loader), [`less-loader`](https://github.com/webpack-contrib/less-loader) uses this to recompile whenever any imported `css` file changes.
+
+### this.addMissingDependency
+
+```typescript
+addMissingDependency(file: string)
+```
+
+Add a non-existing file as a dependency of the loader result in order to make them watchable. Similar to `addDependency`, but handles the creation of files during compilation before watchers are attached correctly.
 
 ### this.async