Add missing PSK identity comment.

Change-Id: I1ca9f252afeea6cdcaa6d75e842eab019c82a7e4
Reviewed-on: https://boringssl-review.googlesource.com/12184
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index 39a6b33..3690548 100644
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -1960,6 +1960,7 @@
     return 0;
   }
 
+  /* We only advertise one PSK identity, so the only legal index is zero. */
   if (psk_id != 0) {
     OPENSSL_PUT_ERROR(SSL, SSL_R_PSK_IDENTITY_NOT_FOUND);
     *out_alert = SSL_AD_UNKNOWN_PSK_IDENTITY;