Added a clarification on intercepting key events. Resolves #181

box.go

@@ -162,6 +162,13 @@ func (b *Box) InputHandler() func(event *tcell.EventKey, setFocus func(p Primiti
 // be called.
 //
 // Providing a nil handler will remove a previously existing handler.
+//
+// Note that this function will not have an effect on primitives composed of
+// other primitives, such as Form, Flex, or Grid. Key events are only captured
+// by the primitives that have focus (e.g. InputField) and only one primitive
+// can have focus at a time. Composing primitives such as Form pass the focus on
+// to their contained primitives and thus never receive any key events
+// themselves. Therefore, they cannot intercept key events.
 func (b *Box) SetInputCapture(capture func(event *tcell.EventKey) *tcell.EventKey) *Box {
 	b.inputCapture = capture
 	return b