Add text describing the changes done in the MR in more detail

(cherry picked from commit 096d41b4e2)
(cherry picked from commit 469286e8eb24ebd6227e65b18eb4c30e13a58877)

CHANGES

@@ -1,3 +1,6 @@
+5516.	[func]		The default EDNS buffer size has been changed from 4096
+			to 1232. [GL #2183]
+
 5513.	[doc]		The ARM section describing the "rrset-order" statement
 			was rewritten to make it unambiguous and up-to-date with
 			the source code. [GL #2139]

doc/arm/notes-9.11.24.xml

@@ -55,7 +55,16 @@
     <itemizedlist>
       <listitem>
         <para>
-          None.
+	  [DNS Flag Day 2020]: The default EDNS buffer size has been changed
+	  from 4096 to 1232.  According to the measurements done by multiple
+	  parties this should not be causing any operational problems as most of
+	  the Internet "core" is able to cope with IP message sizes between
+	  1400-1500 bytes, the 1232 size was picked as a conservative minimal
+	  number that could be changed by the DNS operator to a estimated path
+	  MTU minus the estimated header space. In practice, the smallest MTU
+	  witnessed in the operational DNS community is 1500 octets, the
+	  Ethernet maximum payload size, so a a useful default for maximum
+	  DNS/UDP payload size on reliable networks would be 1400. [GL #2183]
         </para>
       </listitem>
     </itemizedlist>