io/pointer: delete documentation that Cancel events have pointer IDs

Cancel events affect the entire gesture and as such all active pointers.

References: https://todo.sr.ht/~eliasnaur/gio/657
Signed-off-by: Elias Naur <mail@eliasnaur.com>

io/pointer/pointer.go

@@ -19,7 +19,7 @@ type Event struct {
 	Source Source
 	// PointerID is the id for the pointer and can be used
 	// to track a particular pointer from Press to
-	// Release or Cancel.
+	// Release.
 	PointerID ID
 	// Priority is the priority of the receiving handler
 	// for this event.