`any_invocable`: Add public documentation for undefined behavior when invoking…

`any_invocable`: Add public documentation for undefined behavior when invoking an empty AnyInvocable

This is currently documented in `internal/any_invocable.h`, but not in the public API.
For example, `std::function` [documents](https://en.cppreference.com/w/cpp/utility/functional/function) the fact that invoking an empty instance throws `std::bad_function_call`.

PiperOrigin-RevId: 631621604
Change-Id: I6b886a805ffa0e7aaef5f6971e1eafd14f94050c

absl/functional/any_invocable.h

@@ -151,6 +151,12 @@ ABSL_NAMESPACE_BEGIN
 //
 // Attempting to call `absl::AnyInvocable` multiple times in such a case
 // results in undefined behavior.
+//
+// Invoking an empty `absl::AnyInvocable` results in undefined behavior:
+//
+//   // Create an empty instance using the default constructor.
+//   AnyInvocable<void()> empty;
+//   empty();  // WARNING: Undefined behavior!
 template <class Sig>
 class AnyInvocable : private internal_any_invocable::Impl<Sig> {
  private:
@@ -167,6 +173,7 @@ class AnyInvocable : private internal_any_invocable::Impl<Sig> {
   // Constructors
 
   // Constructs the `AnyInvocable` in an empty state.
+  // Invoking it results in undefined behavior.
   AnyInvocable() noexcept = default;
   AnyInvocable(std::nullptr_t) noexcept {}  // NOLINT
 
@@ -277,6 +284,8 @@ class AnyInvocable : private internal_any_invocable::Impl<Sig> {
   // In other words:
   //   std::function<void()> f;  // empty
   //   absl::AnyInvocable<void()> a = std::move(f);  // not empty
+  //
+  // Invoking an empty `AnyInvocable` results in undefined behavior.
   explicit operator bool() const noexcept { return this->HasValue(); }
 
   // Invokes the target object of `*this`. `*this` must not be empty.

absl/functional/internal/any_invocable.h

@@ -39,7 +39,7 @@
 //    target object, directly returning the result.                           //
 //                                                                            //
 // When in the logically empty state, the manager function is an empty        //
-// function and the invoker function is one that would be undefined-behavior  //
+// function and the invoker function is one that would be undefined behavior  //
 // to call.                                                                   //
 //                                                                            //
 // An additional optimization is performed when converting from one           //