[gnome-nettool] Extend comment to clarify what Mailbox Exchange (MX) means

[Germán Poo-Caamaño <gpoo src gnome org>]

commit 62a053db012eccb006ce0b6d3b2edffeac6d20c3
Author: Germán Poo-Caamaño <gpoo gnome org>
Date:   Sun Mar 20 14:58:50 2016 -0700

    Extend comment to clarify what Mailbox Exchange (MX) means
    
    Fix https://bugzilla.gnome.org/show_bug.cgi?id=763957

 src/main.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)
---
diff --git a/src/main.c b/src/main.c
index b003fd1..bb58845 100644
--- a/src/main.c
+++ b/src/main.c
@@ -666,7 +666,12 @@ nettool_lookup_setup_combo_type (Netinfo *lookup)
                N_("Internet Address"),
                N_("Canonical Name"),
                N_("CPU / OS Type"),
-               /* When asking for MX record in DNS context */
+               /* Translators: Mailbox Exchange refers to a DNS record.
+                  It defines the priority of mail servers to contact
+                  to deliver an email.
+                  It has nothing to do with e-mail clients.
+                  See https://tools.ietf.org/html/rfc5321#section-3.6.2
+                */
                N_("Mailbox Exchange"),
                N_("Mailbox Information"),
                /* When asking for NS record in DNS context */