Add some translation context for a technical combo box

Read the German translation today and it nearly started to cry :-(
I hope some context can help to translate minimization not as
ressource wasting.

kcmkwin/kwincompositing/main.ui

@@ -473,17 +473,17 @@
             </property>
             <item>
              <property name="text">
-              <string>Always (Breaks minimization)</string>
+              <string extracomment="A window thumbnail requires to have the corresponding window mapped. To have thumbnails at all time, windows are not unmapped. This can break window minimization as it is modelled as unmapping of windows.">Always (Breaks minimization)</string>
              </property>
             </item>
             <item>
              <property name="text">
-              <string>Only for Shown Windows</string>
+              <string extracomment="Windows are not unmapped if the window is somewhere visible on any of the virtual desktops.">Only for Shown Windows</string>
              </property>
             </item>
             <item>
              <property name="text">
-              <string>Never</string>
+              <string extracomment="Windows are unmapped as they are requested. This can lead to not having updated thumbnials for windows on other desktops.">Never</string>
              </property>
             </item>
            </widget>