Warning staff affinities should only decrease

This warning can appear if there are no staves in the printed output, for example if there are just a ChordName context and a Lyrics context as in a lead sheet. The warning messages can be avoided by making one of the contexts behave as a staff by inserting

\override VerticalAxisGroup.staff-affinity = ##f

at its start. For details, see “Spacing of non-staff lines” in Flexible vertical spacing within systems.