io/system: improve FrameEvent documentation

Phrase stolen from Egon's architecture document.

Replace tabs with spaces in the example while here.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

io/system/system.go

@@ -14,7 +14,8 @@ import (
 )
 
 // A FrameEvent requests a new frame in the form of a list of
-// operations.
+// operations that describes what to display and how to handle
+// input.
 type FrameEvent struct {
 	Config Config
 	// Size is the dimensions of the window.