Mention can_focus in docs, note on how bindings are checked

Author: darrenburns

docs/guide/input.md

@@ -113,20 +113,21 @@ The app splits the screen in to quarters, with a `RichLog` widget in each quarte
 
 You can move focus by pressing the ++tab++ key to focus the next widget. Pressing ++shift+tab++ moves the focus in the opposite direction.
 
+### Focusable widgets
+
+Each widget has a boolean `can_focus` attribute which determines if it is capable of receiving focus.
+Note that `can_focus=True` does not mean the widget will _always_ be focusable.
+For example, a disabled widget cannot receive focus even if `can_focus` is `True`.
+
 ### Controlling focus
 
 Textual will handle keyboard focus automatically, but you can tell Textual to focus a widget by calling the widget's [focus()][textual.widget.Widget.focus] method.
+By default, Textual will focus the first focusable widget when the app starts.
 
 ### Focus events
 
 When a widget receives focus, it is sent a [Focus](../events/focus.md) event. When a widget loses focus it is sent a [Blur](../events/blur.md) event.
 
-### Focusable widgets
-
-Each widget has a boolean `can_focus` attribute which determines if it is capable of receiving focus.
-Note that `can_focus=True` does not mean the widget will _always_ be focusable.
-For example, a disabled widget cannot receive focus even if `can_focus` is `True`.
-
 ## Bindings
 
 Keys may be associated with [actions](../guide/actions.md) for a given widget. This association is known as a key _binding_.
@@ -160,6 +161,9 @@ Note how the footer displays bindings and makes them clickable.
     Multiple keys can be bound to a single action by comma-separating them.
     For example, `("r,t", "add_bar('red')", "Add Red")` means both ++r++ and ++t++ are bound to `add_bar('red')`.
 
+When you press a key, Textual will first check for a matching binding in the `BINDINGS` list of the currently focused widget.
+If no match is found, it will search upwards through the DOM all the way up to the `App` looking for a match.
+
 ### Binding class
 
 The tuple of three strings may be enough for simple bindings, but you can also replace the tuple with a [Binding][textual.binding.Binding] instance which exposes a few more options.