Add docs for @notUndefined decorator and its usage

markdown-pages/docs/manual/module.mdx

@@ -350,6 +350,12 @@ It's also useful to hide the underlying type as an implementation detail others
 can't rely on. If you ask what the type of `Company.profession` is, instead of
 exposing the variant, it'll only tell you "it's `Company.profession`".
 
+This also means that the compiler can't make assumptions about the type.
+In certain cases, when working with abstract types and `option` for example, the compiler doesn't know whether
+the abstract type can be the JavaScript value `undefined` or not. This can lead to less optimal code being generated.
+For this reason, you can use the `@notUndefined` decorator to tell the compiler that the abstract type can never be `undefined`
+(use with caution and see the `@notUndefined` decorator documentation for more details and caveats).
+
 ### Extending module signatures
 
 Like modules themselves, module signatures can also be extended by other module signatures using `include`. Again, **heavily discouraged**:

markdown-pages/syntax-lookup/decorator_non_undefined.mdx

@@ -0,0 +1,35 @@
+---
+id: "not-undefined-decorator"
+keywords: ["not", "undefined", "decorator"]
+name: "@notUndefined"
+summary: "This is the `@notUndefined` decorator."
+category: "decorators"
+---
+
+**Since 12.0**
+
+Use `@notUndefined` on an abstract type to tell the compiler that
+values of that type can never be JavaScript `undefined`.
+
+With `@notUndefined`, the compiler can treat the abstract type as never-undefined for optimization purposes (notably option representation and code generation decisions).
+
+> **Note:** This attribute is a compiler contract. If `undefined` can flow into that type through unsafe interop or casts, generated code assumptions can be broken. Use it only when this guarantee is actually true.
+
+### Example
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+```res
+@notUndefined
+type t
+```
+
+```js
+// Empty output
+```
+
+</CodeTab>
+
+### References
+
+- [Signatures](../docs/manual/module.mdx#signatures)