upgrade: Stabilize deployment staging

We're waaay overdue for this, it's been the default
in rpm-ostree for years, and solves several important bugs
around not capturing `/etc` while things are running.

Also, `ostree admin upgrade --stage` (should) become idempotent.

Closes: https://github.com/ostreedev/ostree/issues/2389

man/ostree-admin-upgrade.xml

@@ -96,6 +96,16 @@ Boston, MA 02111-1307, USA.
                 and is ready to deploy them.</para></listitem>
             </varlistentry>
 
+            <varlistentry>
+                <term><option>--stage</option></term>
+
+                <listitem><para>
+                    Perform deployment finalization at shutdown time.  Recommended,
+                    and will likely become the default in the future.
+                </para></listitem>
+            </varlistentry>
+            
+
             <varlistentry>
                 <term><option>--reboot</option>,<option>-r</option></term>