widget: Allow underallocation for scrollables

Also document this fact.

gtk/gtkscrollable.c

@@ -45,6 +45,14 @@
  *   </listitem>
  *   <listitem>
  *     <para>
+ *       Because its preferred size is the size for a fully expanded widget,
+ *       the scrollable widget must be able to cope with underallocations.
+ *       This means that it must accept any value passed to its
+ *       #GtkWidgetClass.size_allocate() function.
+ *     </para>
+ *   </listitem>
+ *   <listitem>
+ *     <para>
  *       When the parent allocates space to the scrollable child widget, the widget should update
  *       the adjustments' properties with new values.
  *     </para>