Document requirement for explicit default in Enum properties

When using the `Property` macro with enum types that derive `Enum`, the
default value must be explicitly set using `#[property(builder(...))]`.

Author: nozwock

glib-macros/src/lib.rs

@@ -525,6 +525,34 @@ pub fn closure_local(item: TokenStream) -> TokenStream {
 /// }
 /// ```
 ///
+/// When using [`Properties`] with an enum that derives [`Enum`], the default
+/// value must be explicitly set via the `builder` parameter of the
+/// `#[property]` attribute.
+///
+/// ```ignore
+/// use glib::prelude::*;
+/// use glib::subclass::prelude::*;
+///
+/// #[derive(Debug, Copy, Clone, PartialEq, Eq, glib::Enum)]
+/// #[enum_type(name = "MyEnum")]
+/// enum MyEnum {
+///     Val,
+///     // ...
+/// }
+///
+/// mod imp {
+///     use std::cell::Cell;
+///
+///     #[derive(Debug, Default, glib::Properties)]
+///     #[properties(wrapper_type = super::MyObject)]
+///     pub MyObject {
+///         #[property(get, set, builder(MyEnum::Val))]
+///         my_enum: Cell<MyEnum>,
+///         // ...
+///     }
+/// }
+/// ```
+///
 /// An enum can be registered as a dynamic type by setting the derive macro
 /// helper attribute `enum_dynamic`:
 ///