FlowBox: Explain how to avoid inflated min size

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