document APT::Periodic::RandomSleep

The documentation in the patch is from
https://help.ubuntu.com/community/AutomaticSecurityUpdates

That page is licensed under Creative Commons Attribution-ShareAlike
3.0. Because I'm unsure how that license meshes with apt's license
I've not copied the text but formulated the same information freely
in my own words.

The original text was contributed by Chris Bainbridge [1][3]
and Kees Cook [2]. Thanks to them.

[1] https://help.ubuntu.com/community/AutomaticSecurityUpdates?action=diff&rev1=40&rev2=41
[2] https://help.ubuntu.com/community/AutomaticSecurityUpdates?action=diff&rev1=38&rev2=39
[3] https://help.ubuntu.com/community/AutomaticSecurityUpdates?action=diff&rev1=37&rev2=38

Closes: 776380
Thanks: Chris Bainbridge and Kees Cook for initial text

debian/apt.cron.daily

@@ -68,6 +68,19 @@
 #      1:  progress report       (actually any string)
 #      2:  + command outputs     (remove -qq, remove 2>/dev/null, add -d)
 #      3:  + trace on            
+#
+#  APT::Periodic::RandomSleep "1800";
+#  - The apt cron job will delay its execution by a random
+#    time span between zero and 'APT::Periodic::RandomSleep'
+#    seconds.
+#    This is done because otherwise everyone would access the
+#    mirror servers at the same time and put them collectively
+#    under very high strain.
+#    You can set this to '0' if you are using a local mirror and
+#    do not care about the load spikes.
+#    Note that sleeping in the apt job will be delaying the
+#    execution of all subsequent cron.daily jobs.
+#
 
 check_stamp()
 {