The notification is supposed to read "Reboot when it is convenient - to
install the updates", but I kept reading it as "Reboot - when it is
convenient to install the updates.", and I couldn't figure out the
"correct" punctuation to do that while remaining proper English. So move
the wording about instead.

And document it, along with other installation instructions.

So when you do `make PREFIX=/usr` and then `make install`, it installs
to `/usr`.

Checking for new updates isn't resource-intensive, and the downloads
will happen one way or another - this just makes them happen sooner.

The main change here is that the desktop notification will happen
sooner, and once per hour. I think this is reasonable, as XDG
notifications don't interrupt the user's workflow, don't steal focus,
and don't even need to be dismissed. It's just a heads-up, and a
low-level reminder.

Pipe success is determined by the success of the last command in the
pipe, and `grep` indicates failure if no lines were selected. If we
don't download anything then `grep` fails, the pipe fails, and the
script - due to `set -e` - then fails/aborts at that point.

So pipe through `cat` last, which always succeeds.

Because if the network isn't available, it causes an abort. More details
in the README.md

Don't care about the "Reading package lists..." and other interactive
fripperies, or the list of packages no longer required, or the full list
of packages to be upgraded, every time we just want to download new
updates. But logging the actual downloads is kinda useful, so keep
those.