reflect: add comment for String method of Kind struct

On reflect documentation page only this function
doesn't have description, this commit add simple description.

Change-Id: Idcda89ddd1f6fdd1938c4030e89ebdc186255ce6
GitHub-Last-Rev: 1553b834bb4f7a49efc7ff81763a255bc51bbf17
GitHub-Pull-Request: golang/go#28818
Reviewed-on: https://go-review.googlesource.com/c/149721Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/reflect/type.go

@@ -593,6 +593,7 @@ const (
 	kindMask        = (1 << 5) - 1
 )
 
+// String returns the name of k.
 func (k Kind) String() string {
 	if int(k) < len(kindNames) {
 		return kindNames[k]