DISTRO_PORTING: document that distros may/should change fallback DNS as well as fallback NTP if they wish

The DNS and NTP fallback server situation is pretty similar, and
downstream distros might want to change both to whatever they need,
hence mention them both.

DISTRO_PORTING

@@ -15,6 +15,7 @@ HOWTO:
             -D setfont-path=
             -D tty-gid=
             -D ntp-servers=
+            -D dns-servers=
             -D support-url=
 
         2) Try it out. Play around (as an ordinary user) with
@@ -24,11 +25,10 @@ HOWTO:
         This will also inform you about ordering loops and suchlike.
 
 NTP POOL:
-
-        By default, timesyncd uses the Google Public NTP servers
-        time[1-4].google.com. They serve time that uses a leap second
-        smear, and can be up to .5s off from servers that use stepped
-        leap seconds.
+        By default, systemd-timesyncd uses the Google Public NTP servers
+        time[1-4].google.com, if no other NTP configuration is available. They
+        serve time that uses a leap second smear, and can be up to .5s off from
+        servers that use stepped leap seconds.
 
         https://developers.google.com/time/smear
 
@@ -39,6 +39,17 @@ NTP POOL:
 
         http://www.pool.ntp.org/en/vendors.html
 
+        Use -D ntp-servers= to direct systemd-timesyncd to different fallback
+        NTP servers.
+
+DNS SERVERS:
+        By default, systemd-resolved uses the Google Public DNS servers
+        8.8.8.8, 8.8.4.4, 2001:4860:4860::8888, 2001:4860:4860::8844 as
+        fallback, if no other DNS configuration is available.
+
+        Use -D dns-servers= to direct systemd-resolved to different fallback
+        DNS servers.
+
 PAM:
         The default PAM config shipped by systemd is really bare bones.
         It does not include many modules your distro might want to enable
@@ -50,7 +61,6 @@ PAM:
         instead install your own.
 
 CONTRIBUTING UPSTREAM:
-
         We generally do no longer accept distribution-specific patches to
         systemd upstream. If you have to make changes to systemd's source code
         to make it work on your distribution, unless your code is generic