Document suspend button in system menu (#16704)

And improve the whole 'system menu' section.

wiki/src/doc/first_steps/introduction_to_gnome_and_the_tails_desktop.mdwn

@@ -179,29 +179,36 @@ input method for non-Latin scripts.
 System menu
 -----------
 
-In the top-right corner of the top bar, the system menu allows you to
-manage your systems settings and your computer.
+In the top-right corner of the top navigation bar, the system menu allows you to
+manage your settings, connect to a Wi-Fi network, and restart your computer.
 
 [[!img system.png link=no alt=""]]
 
-<div class="next">
+### Networking
 
-<p>See also the documentation on [[connecting to the network|anonymous_internet/networkmanager]].</p>
+From the system menu, you can choose which Wi-Fi network to connect to.
 
-</div>
+See also the documentation on [[connecting to the network|anonymous_internet/networkmanager]].
+
+### System settings
+
+Click on the [[!img lib/preferences-system.png alt="Settings" class="symbolic" link="no"]]
+button to edit your system settings.
+
+These settings won't saved when you restart Tails.
 
 <a id="screen-locker"></a>
 
-### Locking your screen
+### Screen locker
 
-Click on the <span class="button">[[!img lib/network-wireless-encrypted.png alt="Lock" class="symbolic" link="no"]]</span>
-button in the system menu to lock your screen with a password.
+Click on the [[!img lib/network-wireless-encrypted.png alt="Lock Screen" class="symbolic" link="no"]]
+button to lock your screen with a password.
 
 - If you set up an [[administration password|first_steps/startup_options/administration_password]]
   when starting Tails, you can unlock your screen with your administration
   password.
 
-  <div class="note">
+  <div class="tip">
   <p>Your screen will automatically lock after some time if you have set up an
   administration password. To disable this behavior, execute the following
   command:</p>
@@ -215,6 +222,41 @@ button in the system menu to lock your screen with a password.
 
   [[!img screen-locker.png alt="" link="no"]]
 
+### Suspend
+
+Click on the [[!img lib/media-playback-pause.png alt="Suspend" class="symbolic" link="no"]]
+button to suspend your computer.
+
+When suspended, your computer is not computing anymore but is still powered on,
+like if it was standing still but still alive.
+
+To resume from suspend and go back to the Tails desktop, push the power button
+of your computer.
+
+<div class="caution">
+
+<p>If you remove your Tails USB stick while suspended, the computer won't
+shut down immediately and will only shut down when resuming.</p>
+
+<p>An attacker with physical access to your computer and capable of finding
+your screen locker password or setting up a [[cold-boot
+attack|doc/advanced_topics/cold_boot_attacks]] can compromise your Tails while
+suspended.</p>
+
+<p>For more security, [[shut down Tails|doc/first_steps/shutdown]] entirely.</p>
+
+</div>
+
+### Restart
+
+Click on the [[!img lib/view-refresh.png alt="Restart" class="symbolic" link="no"]]
+button to restart your computer.
+
+### Shutdown
+
+Click on the [[!img lib/system-shutdown.png alt="Power Off" class="symbolic" link="no"]]
+button to shut down your computer.
+
 <a id="activities"></a>
 
 Activities overview