bevy_reflect: Mention FunctionRegistry in bevy_reflect::func docs (#15147)

# Objective

The module docs for `bevy_reflect::func` don't mention the
`FunctionRegistry`.

## Solution

Add a section about the `FunctionRegistry` to the module-level
documentation.

## Testing

You can test locally by running:

```
cargo test --doc --package bevy_reflect --all-features
```

crates/bevy_reflect/src/func/mod.rs

@@ -57,7 +57,7 @@
 //! Closures, on the other hand, are special functions that do capture their environment.
 //! These are always defined with anonymous function syntax.
 //!
-//! ```rust
+//! ```
 //! // A closure that captures an immutable reference to a variable
 //! let c = 123;
 //! let add = |a: i32, b: i32| a + b + c;
@@ -94,6 +94,35 @@
 //! For other functions that don't conform to one of the above signatures,
 //! [`DynamicFunction`] and [`DynamicFunctionMut`] can instead be created manually.
 //!
+//! # Function Registration
+//!
+//! This module also provides a [`FunctionRegistry`] that can be used to register functions and closures
+//! by name so that they may be retrieved and called dynamically.
+//!
+//! ```
+//! # use bevy_reflect::func::{ArgList, FunctionRegistry};
+//! fn add(a: i32, b: i32) -> i32 {
+//!     a + b
+//! }
+//!
+//! let mut registry = FunctionRegistry::default();
+//!
+//! // You can register functions and methods by their `std::any::type_name`:
+//! registry.register(add).unwrap();
+//!
+//! // Or you can register them by a custom name:
+//! registry.register_with_name("mul", |a: i32, b: i32| a * b).unwrap();
+//!
+//! // You can then retrieve and call these functions by name:
+//! let reflect_add = registry.get(std::any::type_name_of_val(&add)).unwrap();
+//! let value = reflect_add.call(ArgList::default().push_owned(10_i32).push_owned(5_i32)).unwrap();
+//! assert_eq!(value.unwrap_owned().try_downcast_ref::<i32>(), Some(&15));
+//!
+//! let reflect_mul = registry.get("mul").unwrap();
+//! let value = reflect_mul.call(ArgList::default().push_owned(10_i32).push_owned(5_i32)).unwrap();
+//! assert_eq!(value.unwrap_owned().try_downcast_ref::<i32>(), Some(&50));
+//! ```
+//!
 //! [`PartialReflect`]: crate::PartialReflect
 //! [`Reflect`]: crate::Reflect
 //! [lack of variadic generics]: https://poignardazur.github.io/2024/05/25/report-on-rustnl-variadics/