reflect: expand doc for Value.Interface

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5716057

src/pkg/reflect/value.go

@@ -800,13 +800,15 @@ func (v Value) CanInterface() bool {
 	return v.flag&(flagMethod|flagRO) == 0
 }
 
-// Interface returns v's value as an interface{}.
+// Interface returns v's current value as an interface{}.
+// It is equivalent to:
+//	var i interface{} = (v's underlying value)
 // If v is a method obtained by invoking Value.Method
 // (as opposed to Type.Method), Interface cannot return an
 // interface value, so it panics.
 // It also panics if the Value was obtained by accessing
 // unexported struct fields.
-func (v Value) Interface() interface{} {
+func (v Value) Interface() (i interface{}) {
 	return valueInterface(v, true)
 }