language: improve docs for Parent

Fixes golang/go#25603

Change-Id: Ie94650ed2579619abfdbbc7410c2f400dec88b25
Reviewed-on: https://go-review.googlesource.com/c/145437
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Ross Light <light@google.com>

language/examples_test.go

diff --git a/language/examples_test.go b/language/examples_test.go
index 68caa3f..ad089a3 100644
--- a/language/examples_test.go
+++ b/language/examples_test.go
@@ -205,7 +205,7 @@
 	// ac-u: ill-formed
 }
 
-func ExampleParent() {
+func ExampleTag_Parent() {
 	p := func(tag string) {
 		fmt.Printf("parent(%v): %v\n", tag, language.Make(tag).Parent())
 	}

language/language.go

diff --git a/language/language.go b/language/language.go
index b042be3..b939c89 100644
--- a/language/language.go
+++ b/language/language.go
@@ -335,6 +335,11 @@
 // Parent returns the CLDR parent of t. In CLDR, missing fields in data for a
 // specific language are substituted with fields from the parent language.
 // The parent for a language may change for newer versions of CLDR.
+//
+// Parent returns a tag for a less specific language that is mutually
+// intelligible or Und if there is no such language. This may not be the same as
+// simply stripping the last BCP 47 subtag. For instance, the parent of "zh-TW"
+// is "zh-Hant", and the parent of "zh-Hant" is "und".
 func (t Tag) Parent() Tag {
 	return Tag(compact.Tag(t).Parent())
 }