[11.x] Add documentation for Context::missing() (#10154)

* add documentation for missing method

* Update context.md

---------

Co-authored-by: Taylor Otwell <[email protected]>

Author: vbergerondev

context.md

@@ -250,14 +250,18 @@ $data = Context::all();
 <a name="determining-item-existence"></a>
 ### Determining Item Existence
 
-You may use the `has` method to determine if the context has any value stored for the given key:
+You may use the `has` and `missing` methods to determine if the context has any value stored for the given key:
 
 ```php
 use Illuminate\Support\Facades\Context;
 
 if (Context::has('key')) {
     // ...
 }
+
+if (Context::missing('key')) {
+    // ...
+}
 ```
 
 The `has` method will return `true` regardless of the value stored. So, for example, a key with a `null` value will be considered present: