Write documentation for the `debug` feature

src/application.rs

@@ -14,6 +14,9 @@ use crate::{window, Command, Element, Executor, Settings, Subscription};
 /// you do not intend to perform any background work in your program, the
 /// [`Sandbox`](trait.Sandbox.html) trait offers a simplified interface.
 ///
+/// When using an [`Application`] with the `debug` feature enabled, a debug view
+/// can be toggled by pressing `F12`.
+///
 /// [`Application`]: trait.Application.html
 ///
 /// # Examples

winit/src/application.rs

@@ -12,6 +12,9 @@ use crate::{
 ///
 /// An [`Application`](trait.Application.html) can execute asynchronous actions
 /// by returning a [`Command`](struct.Command.html) in some of its methods.
+///
+/// When using an [`Application`] with the `debug` feature enabled, a debug view
+/// can be toggled by pressing `F12`.
 pub trait Application: Sized {
     /// The graphics backend to use to draw the [`Application`].
     ///