FlowBox: Explain how to avoid inflated min size

along the orthogonal orientation. It seems a FlowBox on its own can only
handle being shrunk along its main orientation. The orthogonal requests
a huge min size – reserving what it would need if the main orientation
got its min size, which would flow all children in 1 line orthogonally.

Adding it to a ScrolledWindow (any policy) enables free shrinking, so
size_allocate() can reflow how users in this situation probably expect.

https://bugzilla.gnome.org/show_bug.cgi?id=787021

gtk/gtkflowbox.c

@@ -41,6 +41,10 @@
  * Reducing the height will require more columns, so a larger width will
  * be requested.
  *
+ * The size request of a GtkFlowBox alone may not be what you expect; if you
+ * need to be able to shrink it along both axes and dynamically reflow its
+ * children, you may have to wrap it in a #GtkScrolledWindow to enable that.
+ *
  * The children of a GtkFlowBox can be dynamically sorted and filtered.
  *
  * Although a GtkFlowBox must have only #GtkFlowBoxChild children,