Add comment on the types of extensible builders

Author: soareschen

content/blog/2025-10-12-v0.5.0-release.md

@@ -175,9 +175,9 @@ pub struct FooBar {
 You can now construct a `FooBar` instance while letting the `bar` field fall back to its default value of `0`:
 
 ```rust
-let foo_bar = FooBar::builder()
-    .build_field(PhantomData::<Symbol!("foo")>, "foo".to_owned())
-    .finalize_with_default();
+let foo_bar = FooBar::builder() // __PartialFooBar<IsNothing, IsNothing>
+    .build_field(PhantomData::<Symbol!("foo")>, "foo".to_owned()) // __PartialFooBar<IsPresent, IsNothing>
+    .finalize_with_default(); // FooBar
 ```
 
 Behind the scene, `finalize_with_default` works by using the `UpdateField` trait to perform a *natural transformation* on each field modifier. It applies the `Default` implementation to convert `IsNothing` fields into `IsPresent`, completing the record automatically.
@@ -193,11 +193,11 @@ To address this limitation, a new `IsOptional` field state has been introduced.
 You can create and use an optional builder with the `optional_builder`, `set`, and `finalize_optional` methods as shown below:
 
 ```rust
-let foo_bar = FooBar::optional_builder()
-    .set(PhantomData::<Symbol!("foo")>, "foo".to_owned())
-    .set(PhantomData::<Symbol!("bar")>, 42)
-    .finalize_optional()
-    .unwrap()
+let foo_bar = FooBar::optional_builder() // __PartialFooBar<IsOptional, IsOptional>
+    .set(PhantomData::<Symbol!("foo")>, "foo".to_owned()) // __PartialFooBar<IsOptional, IsOptional>
+    .set(PhantomData::<Symbol!("bar")>, 42) // __PartialFooBar<IsOptional, IsOptional>
+    .finalize_optional() // Result<FooBar, String>
+    .unwrap() // FooBar
 ```
 
 Unlike the original typestate builder, the type of the builder remains `__PartialFooBar<IsOptional, IsOptional>` after each call to `set`. The `finalize_optional` method returns a `Result`, producing an error if any field contains a `None` value. This check is necessary because, without the typestate guarantees, the compiler cannot ensure at compile time that all fields are initialized.