docs: document the [required] struct attribute (#6956)

doc/docs.md

@@ -1139,6 +1139,21 @@ Array and map fields are allocated.
 
 It's also possible to define custom default values.
 
+### Required fields
+
+```v
+struct Foo {
+    n int [required]
+}
+```
+
+You can mark a struct field with the `[required]` attribute, to tell V that
+that field must be initialized when creating an instance of that struct.
+
+This example will not compile, since the field `n` isn't explicitly initialized:
+```v failcompile
+_ = Foo{}
+```
 
 <a id='short-struct-initialization-syntax' />