reflect: document reflect.TypeOf((*Foo)(nil)).Elem() idiom

See also golang-dev discussion:
https://groups.google.com/d/msg/golang-dev/Nk9gnTINlTg/SV8rBt-2__kJ

Change-Id: I49edd98d73400c1757b6085dec86752de569c01a
Reviewed-on: https://go-review.googlesource.com/8923Reviewed-by: Rob Pike <r@golang.org>

src/reflect/example_test.go

@@ -6,6 +6,8 @@ package reflect_test
 
 import (
 	"fmt"
+	"io"
+	"os"
 	"reflect"
 )
 
@@ -64,3 +66,16 @@ func ExampleStructTag() {
 	// Output:
 	// blue gopher
 }
+
+func ExampleTypeOf() {
+	// As interface types are only used for static typing, a
+	// common idiom to find the reflection Type for an interface
+	// type Foo is to use a *Foo value.
+	writerType := reflect.TypeOf((*io.Writer)(nil)).Elem()
+
+	fileType := reflect.TypeOf((*os.File)(nil))
+	fmt.Println(fileType.Implements(writerType))
+
+	// Output:
+	// true
+}

src/reflect/type.go

@@ -1009,8 +1009,8 @@ func (t *structType) FieldByName(name string) (f StructField, present bool) {
 	return t.FieldByNameFunc(func(s string) bool { return s == name })
 }
 
-// TypeOf returns the reflection Type of the value in the interface{}.
-// TypeOf(nil) returns nil.
+// TypeOf returns the reflection Type that represents the dynamic type of i.
+// If i is a nil interface value, TypeOf returns nil.
 func TypeOf(i interface{}) Type {
 	eface := *(*emptyInterface)(unsafe.Pointer(&i))
 	return toType(eface.typ)