needrestart: better document Ubuntu-specific behaviours

[Simon Chopin]

[Impact]
As evidenced in https://lists.ubuntu.com/archives/ubuntu-devel-discuss/2024-June/019706.html the Ubuntu customizations to needrestart behaviour need to be much better documented, at least in the default configuration file and the CLI manpage/--help blurb, to try and minimize the amount of user surprise and uproar.

[Test plan]
# Check the contents of /etc/needrestart/needrestart.conf for 'UBUNTU' strings
man needrestart # that should also mention Ubuntu
needrestart --help # likewise
cat /usr/share/doc/needrestart/README.Ubuntu

[ Where problems could occur ]
This is pretty safe, unless someone actually parses out the needrestart --help somewhere?

[Launchpad Janitor (janitor)]

This bug was fixed in the package needrestart - 3.6-8ubuntu3

---------------
needrestart (3.6-8ubuntu3) oracular; urgency=medium

  * d/p/ubuntu-mode.patch: Don't touch /run/reboot-required on kernel updates
    (LP: #2065863)
  * Ubuntu mode: disable it if restart mode has been explicitly set
    (LP: #2068543)
  * Add some inline documentation for the Ubuntu mode (LP: #2068573)
  * Don't restart the google-guest-agent service (LP: #2063442)
  * Don't restart the GH runner provisioner (LP: #2067800)
  * tests: guard against looping when failing.

 -- Simon Chopin <email address hidden> Fri, 14 Jun 2024 11:57:11 +0200