...Add Render Caching tooltip

Forum thread:  
https://forum.freecadweb.org/viewtopic.php?f=17&t=39815&p=338357#p338357

src/Gui/DlgSettings3DView.ui

@@ -165,6 +165,15 @@ is enabled when seeking support on the FreeCAD forums.</string>
         </item>
         <item>
          <widget class="QComboBox" name="renderCache">
+           <property name="toolTip">
+            <string>'Render Caching' is another way to say 'Rendering Acceleration'.
+There are 3 options availble to achieve this: 
+1) 'Auto' (default), let Coin3D decide where to cache.
+2) 'Distributed', manually turn on cache for all view provider root node.
+3) 'Centralized', manually turn off cache in all nodes of all view provider, and
+only cache at the scene graph root node. This offers the fastest rendering speed
+but slower response to any scene changes.</string>
+           </property>
           <property name="currentIndex">
            <number>0</number>
           </property>