Say an update is running before it starts, not only after

Asked what happens if someone shuts the laptop down mid-update, the honest
answer turned out to be poor. The inhibitor does stop them - logind refuses and
falls back to org.freedesktop.login1.power-off-ignore-inhibit, which is
auth_admin_keep - so the desktop answers with an administrator password prompt
reading "Power off the system while an application is inhibiting this". systemd
ships that string untranslated, it never mentions updates, and no KDE catalog
contains any shutdown-blocked text at all. Only systemctl names the reason.

So a notification now goes out before the transaction, asking for the machine to
be left on. Notifications gained a tag: the result replaces the start message in
place rather than stacking a second, contradictory bubble beside it.

Tagged notifications also carry a queue flag. "An update is starting" is only
meaningful while somebody is looking, so unlike the result it is not spooled for
delivery at next login.

doc/cachy-auto-update.1.scd

@@ -102,10 +102,23 @@ restart necessary, a notification says so.
 
 # INTERRUPTED UPDATES
 
+Before a transaction starts, a notification says an update is running and asks
+for the machine to be left on. It is replaced in place by the result once the
+run finishes.
+
 While a transaction is running, *cachy-auto-update* holds a
 *systemd-inhibit*(1) lock on _sleep_ and _shutdown_ in blocking mode, so a
 suspend, a lid close or a normal shutdown request cannot cut it short.
 
+What a user sees when they try anyway is worth knowing: logind refuses the
+request and falls back to the polkit action
+_org.freedesktop.login1.power-off-ignore-inhibit_, which is _auth_admin_keep_,
+so the desktop presents an administrator password prompt reading "Power off the
+system while an application is inhibiting this". That string is shipped
+untranslated by systemd and does not mention updates, and no KDE dialog
+explains the situation either - only *systemctl*(1) names the inhibitor and its
+reason. Hence the notification.
+
 A hard power-off - holding the power button, or losing mains power - is not
 preventable. On the next run a leftover _/var/lib/pacman/db.lck_ is removed if
 it is older than the current boot, since no process able to hold it can still