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 #4514.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/7235080

src/pkg/reflect/type.go

@@ -708,7 +708,7 @@ type StructField struct {
 	Tag       StructTag // field tag string
 	Offset    uintptr   // offset within struct, in bytes
 	Index     []int     // index sequence for Type.FieldByIndex
-	Anonymous bool      // is an anonymous field
+	Anonymous bool      // is an embedded field
 }
 
 // A StructTag is the tag string in a struct field.