deny(missing_docs) for bevy_derive (#19483)

# Objective

Deny missing docs in bevy_derive, towards
https://github.com/bevyengine/bevy/issues/3492.

crates/bevy_derive/src/lib.rs

@@ -1,4 +1,5 @@
-#![expect(missing_docs, reason = "Not all docs are written yet, see #3492.")]
+//! Assorted proc macro derive functions.
+
 #![forbid(unsafe_code)]
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![doc(
@@ -188,11 +189,34 @@ pub fn derive_deref_mut(input: TokenStream) -> TokenStream {
     derefs::derive_deref_mut(input)
 }
 
+/// Generates the required main function boilerplate for Android.
 #[proc_macro_attribute]
 pub fn bevy_main(attr: TokenStream, item: TokenStream) -> TokenStream {
     bevy_main::bevy_main(attr, item)
 }
 
+/// Adds `enum_variant_index` and `enum_variant_name` functions to enums.
+///
+/// # Example
+///
+/// ```
+/// use bevy_derive::{EnumVariantMeta};
+///
+/// #[derive(EnumVariantMeta)]
+/// enum MyEnum {
+///     A,
+///     B,
+/// }
+///
+/// let a = MyEnum::A;
+/// let b = MyEnum::B;
+///
+/// assert_eq!(0, a.enum_variant_index());
+/// assert_eq!("A", a.enum_variant_name());
+///
+/// assert_eq!(1, b.enum_variant_index());
+/// assert_eq!("B", b.enum_variant_name());
+/// ```
 #[proc_macro_derive(EnumVariantMeta)]
 pub fn derive_enum_variant_meta(input: TokenStream) -> TokenStream {
     enum_variant_meta::derive_enum_variant_meta(input)