misc/ios: document the external binary and the required env variables

Change-Id: I1ec2460758b19e5315be061033c1bb5ed9ead4a8
Reviewed-on: https://go-review.googlesource.com/9688
Reviewed-by: Minux Ma <minux@golang.org>
diff --git a/misc/ios/go_darwin_arm_exec.go b/misc/ios/go_darwin_arm_exec.go
index 402de3a..debd2cd 100644
--- a/misc/ios/go_darwin_arm_exec.go
+++ b/misc/ios/go_darwin_arm_exec.go
@@ -59,8 +59,15 @@
 		log.Fatal("usage: go_darwin_arm_exec a.out")
 	}
 
+	// e.g. B393DDEB490947F5A463FD074299B6C0AXXXXXXX
 	devID = getenv("GOIOS_DEV_ID")
+
+	// e.g. Z8B3JBXXXX.org.golang.sample, Z8B3JBXXXX prefix is available at
+	// https://developer.apple.com/membercenter/index.action#accountSummary as Team ID.
 	appID = getenv("GOIOS_APP_ID")
+
+	// e.g. Z8B3JBXXXX, available at
+	// https://developer.apple.com/membercenter/index.action#accountSummary as Team ID.
 	teamID = getenv("GOIOS_TEAM_ID")
 
 	var err error