Improve doc comments on the Requestor interface

request.go

@@ -20,9 +20,20 @@ type selReqState int; const (
 // Requestor provices details about the request such as the window that is
 // requesting the selection data, and what targets it accepts.
 type Requestor interface {
-	Window  () *xwindow.Window
-	Choose  (from []Target) (chosen Target, ok bool)
+	// Window returns the window that is requesting the selection data.
+	Window () *xwindow.Window
+
+	// Choose picks target from a slice of available targets and returns
+	// (target, true). If no target was picket, it returns ("", false).
+	Choose (available []Target) (target Target, ok bool)
+
+	// Success is called once the owner responds with data. The data must be
+	// closed once it has been read.
 	Success (Target, io.ReadCloser)
+
+	// Failure is called if the transfer fails at any point, or if there
+	// isn't any data to begin with. In the first case, an error is given.
+	// In the second case, the error will be nil.
 	Failure (error)
 }