cmd/internal/gc: add a comment to esc.go

Change-Id: I19e6542e7d79d60e39d62339da51a827c5aa6d3b
Reviewed-on: https://go-review.googlesource.com/7668Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/gc/esc.go

@@ -231,6 +231,7 @@ type EscState struct {
 
 var tags [16]*string
 
+// mktag returns the string representation for an escape analysis tag.
 func mktag(mask int) *string {
 	switch mask & EscMask {
 	case EscNone,