reflect: explain StructField.Anonymous field as 'embedded'
reflect: explain StructField.Anonymous field as 'embedded'

The spec mostly uses the term embedded.
It's too late to change the field name but at least fix the docs.

Fixes issue 4514.

R=golang-dev, dave
CC=golang-dev
https://codereview.appspot.com/7235080

wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old