Better documentation for Prompt=normal vs lts

[Alkis Georgopoulos]

The documentation in `cat /etc/update-manager/release-upgrades` mentions what Prompt=normal and Prompt=lts mean.

It doesn't cover this difference though:

a) 20.04 installations with Prompt=normal will get upgraded to 22.04 as soon as it's out (April, https://changelogs.ubuntu.com/meta-release).

b) 20.04 installations with Prompt=lts will get upgraded to 22.04 when 22.04.1 is out (August, https://changelogs.ubuntu.com/meta-release-lts).

This is a sane thing to do, but it's not the only possible sane thing to do, it shouldn't be implied.
Please make it explicit by adding a couple of documentation lines in that file.